This reverts commit 67cb4e4e.

The commit introduced new behaviour unconditionally and by default,
which's considered a breaking change. It was decided to add a compat
option for this change. So revert the commit.

NO_DOC=amended an existing doc issue manually. See https://github.com/tarantool/doc/issues/3295

See the docbot request for details.

Closes #5272

@TarantoolBot document
Title: new `bootstrap_strategy` configuration option

Default behaviour of replica set bootstrap, replica recovery when
connecting to remote nodes and replication reconfiguration is changed.
The new behaviour is controlled by the option `bootstrap_strategy`,
which has the default value "auto".

Now `replication_connect_quorum` configuration option takes no effect,
and the effective quorum value for each stage of configuration (quorum
of established connections, quorum of synced nodes) is determined
automatically.

On replica set bootstrap, the nodes will refuse to boot, unless a
majority is reached (this would mean replication_connect_quorum = 3,
when #box.cfg.repilcation is 4 or 5, for example, or
replication_connect_quorum = 2, when #box.cfg.replication is 2 or 3).
Moreover, the bootstrap leader will fail to boot unless it sees that
every connected node chose it as the bootstrap leader.

On new replica join to an existing cluster, the replica will fail to
boot only if it couldn't connect to anyone. As long as at least one
connection is established, the replica will try to join like before.

Moreover, the replica will check that its box.cfg.replication table
contains every registered node in the cluster, thus ensuring that it has
tried to connect to everyone and chose the best bootstrap leader
possible.

On replication reconfiguration on a working instance and recovery from
local WAL files, the node will try to connect to everyone specified in
box.cfg.replication. Any number of connections (even no connections)
will be deemed a success, but the replica will stay in orphan mode until
it is synced with everyone connected.

If you wish to return to the old behavior, a deprecated setting
`bootstrap_strategy` = "legacy" is left for now. With
`bootstrap_strategy` = "legacy", the node behaves exactly like before:
quorum for both connection and synchronisation is determined by
`replication_connect_quorum`, and neither bootstrap leader nor joining
replicas perform any additional checks on bootstrap.

The only observable behaviour of non-zero replication_sync_timeout is
that it delays box.cfg{replication=...} return until either the node is
synced with others or the timeout passes.

If the timeout passes without reaching sync, box.cfg{} is exited and the
node enters "orphan" state, in which it can't write anything until
either a reconfiguration happens or replicaset is finally synced.

While the previous box.cfg{} call is running (probably waiting for
replication_sync_timeout), the user can't issue another box.cfg{} call.

So basically, while giving no guarantees that the node exits box.cfg{}
in fully synced state, the timeout makes reconfiguration harder: even if
the user knows that the sync won't be achieved, he will have to wait
until the full timeout passes in order to reconfigure replication.

Let's make the default value of replication_sync_timeout 0 instead of
300 seconds. The user still may set the timeout to whatever he likes.
Besides, we have recently introduced box.ctl.on_recovery_state triggers,
which have a "synced" event, and this is the new recommended way to wait
until the node is synced with others.

Part-of #5272

@TarantoolBot document
Title: Changed default value for `box.cfg.replication_sync_timeout

The default value for `replication_sync_timeout` configuration option
was changed from 300 seconds to 0.

Now the instance appends a list of registered replica set members it
knows of to its ballot.

Prerequisite #5272

NO_CHANGELOG=not user-visible

@TarantoolBot document
Title: New fields in instance's ballot.

Instance's ballot (a response to IPROTO_VOTE sent on replica connect)
receives two new fields:
1) The uuid of the node this instance considers the bootstrap leader.
   Key: IPROTO_BALLOT_BOOTSTRAP_LEADER_UUID = 0x08
   Value: uuid, encoded as 36-byte string (like
   "bfd2b31c-b740-43e5-bf3c-28538a74c9a6").
2) An array of registered replica set members uuids.
   Key: IPROTO_BALLOT_REGISTERED_REPLICA_UUIDS = 0x09
   Value: a MP_ARRAY of uuids, each uuid encoded as a 36-byte string
   (like in an example above).

Note that bootstrap leader uuid is not set when an anonymous replica
registers, because technically it's not performing a bootstrap.

Prerequisite #5272

NO_DOC=appended to next commit's doc request
NO_CHANGELOG=not user-visible

Add a new builtin event carrying instance's ballot information (that is,
what this instance would normally send in reply to IPROTO_VOTE request).

The event will be watched by connecting replicas to find the bootstrap
leader.

Prerequisite #5272

NO_DOC=technically user-visible, but not intended for users
NO_CHANGELOG=see NO_DOC

luatest_helpers/cluster module was recently added to luatest trunk and
renamed to replica_set.
Let's update its name everywhere in gh_6260_add_builtin_events_test,
since this test will be amended in the following commits and the new
module name will be used.

In-scope-of #5272

NO_DOC=refactoring
NO_CHANGELOG=refactoring

Some internal modules have been recently copied to luatest repo [1] and
now they can be safely removed, and the corresponding functionality from
luatest can be used instead.

Affected modules:

- test/luatest_helpers/cluster.lua

[1] tarantool/luatest#271

Closes tarantool/luatest#237
Closes tarantool/luatest#269

NO_DOC=testing stuff
NO_TEST=testing stuff
NO_CHANGELOG=testing stuff

Some internal modules have been recently copied to luatest repo [1,2]
and now they can be safely removed, and the corresponding functionality
from luatest can be used instead.

Affected modules:

- test/luatest_helpers/server.lua

[1] tarantool/luatest#258
[2] tarantool/luatest#266

Closes tarantool/luatest#239

NO_DOC=testing stuff
NO_TEST=testing stuff
NO_CHANGELOG=testing stuff

Some luatest framework tests use Lua `assert`s, which are incomprehensible
when failed (the only information provided is 'assertion failed!'),
making debugging difficult: replace them with luatest `assert`s and their
context-specific varieties.

NO_CHANGELOG=<code health>
NO_DOC=<code health>

Added predefined system events: box.status, box.id, box.election and
box.schema.

Closes #6260

@TarantoolBot document
Title: Built-in events for pub/sub

Built-in events are needed, first of all, in order to learn who is the
master, unless it is defined in an application specific way. Knowing who
is the master is necessary to send changes to a correct instance, and
probably make reads of the most actual data if it is important. Also
defined more built-in events for other mutable properties like leader
state change, his election role and election term, schema version change
and instance state.

Built-in events have a special naming schema - their name always starts
with box.. The prefix is reserved for built-in events. Creating new events
with this prefix is banned. Below is a list of all the events + their names
and values:

1. box.id
Description - identification of the instance. Changes are extra rare. Some
values never change or change only once. For example, instance UUID never
changes after the first box.cfg. But is not known before box.cfg is called.
Replicaset UUID is unknown until the instance joins to a replicaset or
bootsa new one, but the events are supposed to start working before that -
right at listen launch. Instance numeric ID is known only after
registration. On anonymous replicas is 0 until they are registered
officially.
Value - {
    MP_STR “id”: MP_UINT; box.info.id,
    MP_STR “instance_uuid”: MP_UUID; box.info.uuid,
    MP_STR “replicaset_uuid”: MP_UUID box.info.cluster.uuid,
}

2. box.status
Description - generic blob about instance status. Its most commonly used
and not frequently changed config options and box.info fields.
Value - {
    MP_STR “is_ro”: MP_BOOL box.info.ro,
    MP_STR “is_ro_cfg”: MP_BOOL box.cfg.read_only,
    MP_STR “status”: MP_STR box.info.status,
}

3. box.election
Description - all the needed parts of box.info.election needed to find who
is the most recent writable leader.
Value - {
    MP_STR “term”: MP_UINT box.info.election.term,
    MP_STR “role”: MP_STR box.info.election.state,
    MP_STR “is_ro”: MP_BOOL box.info.ro,
    MP_STR “leader”: MP_UINT box.info.election.leader,
}

4. box.schema
Description - schema-related data. Currently it is only version.
Value - {
    MP_STR “version”: MP_UINT schema_version,
}

Built-in events can't be override. Meaning, users can't be able to call
box.broadcast(‘box.id’, any_data) etc.

The events are available from the very beginning as not MP_NIL. It's
necessary for supported local subscriptions. Otherwise no way to detect
whether an event is even supported at all by this Tarantool version. If
events are broadcast before box.cfg{}, then the following values will
available:
    box.id = {}
    box.schema = {}
    box.status = {}
    box.election = {}

This way the users will be able to distinguish an event being not supported
at all from box.cfg{} being not called yet. Otherwise they would need to
parse _TARANTOOL version string locally and peer_version in netbox.

Example usage:

 * Client:
   ```lua
   conn = net.box.connect(URI)
   -- Subscribe to updates of key 'box.id'
   w = conn:watch('box.id', function(key, value)
       assert(key == 'box.id')
       -- do something with value
   end)
   -- or to updates of key 'box.status'
   w = conn:watch('box.status', function(key, value)
       assert(key == 'box.status')
       -- do something with value
   end)
   -- or to updates of key 'box.election'
   w = conn:watch('box.election', function(key, value)
       assert(key == 'box.election')
       -- do something with value
   end)
   -- or to updates of key 'box.schema'
   w = conn:watch('box.schema', function(key, value)
       assert(key == 'box.schema')
       -- do something with value
   end)
   -- Unregister the watcher when it's no longer needed.
   w:unregister()
   ```