gc/xlog: delay xlog cleanup until relays are subscribed

In case if replica managed to be far behind the master node
(so there are a number of xlog files present after the last
master's snapshot) then once master node get restarted it
may clean up the xlogs needed by the replica to subscribe
in a fast way and instead the replica will have to rejoin
reading a number of data back.

Lets try to address this by delaying xlog files cleanup
until replicas are got subscribed and relays are up
and running. For this sake we start with cleanup fiber
spinning in nop cycle ("paused" mode) and use a delay
counter to wait until relays decrement them.

This implies that if `_cluster` system space is not empty
upon restart and the registered replica somehow vanished
completely and won't ever come back, then the node
administrator has to drop this replica from `_cluster`
manually.

Note that this delayed cleanup start doesn't prevent
WAL engine from removing old files if there is no
space left on a storage device. The WAL will simply
drop old data without a question.

We need to take into account that some administrators
might not need this functionality at all, for this
sake we introduce "wal_cleanup_delay" configuration
option which allows to enable or disable the delay.

Closes #5806

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

@TarantoolBot document
Title: Add wal_cleanup_delay configuration parameter

The `wal_cleanup_delay` option defines a delay in seconds
before write ahead log files (`*.xlog`) are getting started
to prune upon a node restart.

This option is ignored in case if a node is running as
an anonymous replica (`replication_anon = true`). Similarly
if replication is unused or there is no plans to use
replication at all then this option should not be considered.

An initial problem to solve is the case where a node is operating
so fast that its replicas do not manage to reach the node state
and in case if the node is restarted at this moment (for various
reasons, for example due to power outage) then `*.xlog` files might
be pruned during restart. In result replicas will not find these
files on the main node and have to reread all data back which
is a very expensive procedure.

Since replicas are tracked via `_cluster` system space this we use
its content to count subscribed replicas and when all of them are
up and running the cleanup procedure is automatically enabled even
if `wal_cleanup_delay` is not expired.

The `wal_cleanup_delay` should be set to:

 - `0` to disable the cleanup delay;
 - `>= 0` to wait for specified number of seconds.

By default it is set to `14400` seconds (ie `4` hours).

In case if registered replica is lost forever and timeout is set to
infinity then a preferred way to enable cleanup procedure is not setting
up a small timeout value but rather to delete this replica from `_cluster`
space manually.

Note that the option does *not* prevent WAL engine from removing
old `*.xlog` files if there is no space left on a storage device,
WAL engine can remove them in a force way.

Current state of `*.xlog` garbage collector can be found in
`box.info.gc()` output. For example

``` Lua
 tarantool> box.info.gc()
 ---
   ...
   is_paused: false
```

The `is_paused` shows if cleanup fiber is paused or not.