By default cargo runs tests in parallel in multiple threads.  Both
`test_log_level` and `test_parse` access environment variables which are
shared across threads. Consequently, their concurrent modification
results in the test failure.

This patch unites these two tests making it linear.

Close https://git.picodata.io/picodata/picodata/picodata/-/issues/28

1. Lower log level of connection errors in `netork.rs`.
2. Give raft fibers a name.

There were some problems with join requests synchronization. Raft
forbids proposing a configuration change if there's another one
uncommitted (see [1]). In that case, it replaces an `EntryConfChange`
with an `EntryNormal`. It could happen at any time even without bugs in
code due to the network partitioning, and its the repsonsibility of
the picodata product to handle it properly.

Earlier, there was no way to wait when raft leaves the joint state. It
used to slow down cluster assembling and made it race-prone. The waiting
for the cluster readiness is also important in tests. Some operations
(the most important amongst them is leader switching) are impossible
until instance finishes promotion to a voter. For instance, raft rejects
`MsgTimeoutNow` unless the node is promotable (see [2]). It makes some
testing scenarios flaky.

This patch introduces new synchronization primitive - `JointStateLatch`.
The latch is held on the leader and is locked upon
`raw_node.propose_conf_change()`. It's unlocked only when the second
(implicit) conf change that represents leaving joint state is committed.
The latch also tracks the index of the corresponding `EntryConfChange`.
Even if raft ignores it for any reason, the latch is still unlocked as
soon as the committed index exceeds the one of the latch.

[1] https://github.com/tikv/raft-rs/blob/v0.6.0/src/raft.rs#L2014-L2026
[2] https://github.com/tikv/raft-rs/blob/v0.6.0/src/raft.rs#L2314

Close https://git.picodata.io/picodata/picodata/picodata/-/issues/47
Close https://git.picodata.io/picodata/picodata/picodata/-/issues/53

Waiting for a valid `leader_id` on a node isn't enough. It may already
have one, but still be a Learner. Instead, the fixture should wait until
the node is promoted to voter.

Close https://git.picodata.io/picodata/picodata/picodata/-/issues/56

We don't want a child process to live without the supervisor.

Usually, supervisor waits for child forever and retransmits termination
signals. But if the parent is killed with a SIGKILL there's no way to
pass anything.

This patch supplies a child process with a `supervisor_fuse` fiber. It
tries to read from a pipe (that supervisor never writes to), and if the
writing end is closed, it means the supervisor has terminated. In this
case, child process terminates too.

Part of https://git.picodata.io/picodata/picodata/picodata/-/issues/56

This patch reduces the variable passing trace length. Instead of
returning it from function to function, call `exit()` in place.

The trace of exit code before the patch:
```
exit() <- main <- main_run <- fork(child) <- tarantool_main
```

And now:
```
exit() <- tarantool_main
```

Type safety is ensured at comilation time by using `!` type for
`fn main()`. See https://doc.rust-lang.org/reference/types/never.html

```console
$ cargo clippy
   Compiling picodata v0.1.0 (/home/rosik/w/picodata)
warning: 'tarantool-sys/patches-applied' exists, so patching step is
skipped
warning: single-character string constant used as pattern
   --> src/args.rs:194:47
    |
194 |     let (host, port) = match text.rsplit_once(":") {
    |                                               ^^^ help: try using
a `char` instead: `':'`
    |
    = note: `#[warn(clippy::single_char_pattern)]` on by default
    = help: for further information visit
https://rust-lang.github.io/rust-clippy/master/index.html#single_char_pattern

warning: called `cloned().next()` on an `Iterator`. It may be more
efficient to call `next().cloned()` instead
   --> src/discovery.rs:154:27
    |
154 |                 let res =
    peers.difference(&self.visited).cloned().next();
    |
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this:
`peers.difference(&self.visited).next().cloned()`
    |
    = note: `#[warn(clippy::iter_overeager_cloned)]` on by default
    = help: for further information visit
https://rust-lang.github.io/rust-clippy/master/index.html#iter_overeager_cloned

warning: you seem to be trying to use `match` for destructuring a single
pattern. Consider using `if let`
  --> src/ipc.rs:27:9
   |
27 | /         match rmp_serde::encode::write(&mut self.fd, msg) {
28 | |             Err(e) => tlog!(Error, "ipc error: {e}"),
29 | |             Ok(()) => (),
30 | |         }
   | |_________^ help: try this: `if let Err(e) =
rmp_serde::encode::write(&mut self.fd, msg) { tlog!(Error, "ipc error:
{e}") }`
   |
   = note: `#[warn(clippy::single_match)]` on by default
   = help: for further information visit
https://rust-lang.github.io/rust-clippy/master/index.html#single_match

warning: `picodata` (bin "picodata") generated 3 warnings
    Finished dev [unoptimized + debuginfo] target(s) in 1.41s
```

1. Remove it from functions args where it's not necessary.
2. In other places rename it into `supervisor_ipc`.

Since dynamic discovery isn't implemented yet, instance after
`raft_join` has discovery uninitialized. It's not a reason to panic.

Use common `Notify` instead of a cond. It makes the code more clear and
reliable, because each request is tracked individually and can't be
affected by different yields in the main_loop.

Replace macros with function in `traft::node`.

Remove prctl dependency that is not available on mac.

This pach establishes a friendship between discovery and raft_join.

feat: raft peer discovery PoC

chore: prevent C functions from being optimized out

feat: improve peer discovery

fix: fix tests after making instance_id arg mandatory

Smart supevision with fork

Make it work

Under development

IPC messages to supervisor

One more little step: entrypoint enum

Arrange IPC from child to supervisor

Remove tarantool_main macro

Persist snapshot

Fix some fresh bugs

Implement postjoin

Discovery under refactoring

Enhance discovery

Working on discovery

Fix all discovery bugs known so far

Draft join algorithm

Fail applying snapshot

Joining a learner works

Cleanup snapshot generation

Reorganize traft code and call join automatically

Change peer.commit_index type from option to u64

Implement autopromotion to voter

Implement read_index

Take read_index before self-romotion

Cleanup excess logs

Cleanup logs and code

Deep refactoring in progress

Finish refactoring db schema

Embed entries applying inside traft node

Refactor raft node communication

Replace fiber channel with a mailbox, which is a `Vec<_>` + fiber cond.
It allows to batch raft commands in a more predictable way and makes
the code less error-prone.

Remove commented code

Simplify raft nodes interaction over net_box

Eliminate `traft::Message` struct because its internals aren't used.
Instead, serialize `raft::Message` using protobuf.

Batch ConnectionPool requests

1. Send messages in batches.
2. Allow changing connection uri.
3. Close unused connections after `inactivity_timeout`.

Enhance the raft node

1. Collect results from raft node
2. Fix initial bootstrap which used to fail due to fiber race.
3. Wrap raft storage operations in a transaction.

Bump tarantool module

Add documentation draft

Cleanup warnings

Try fixing tests

Fix test_storage_log

Fix test_traft_pool

Fix luatest single and couple

Start fixing threesome test

Implement concurrent join requests handling