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

Also, use brand new `tarantool::proc` macro for stored procedres.

Close https://gitlab.com/picodata/picodata/picodata/-/issues/27

- don't patch if tarantool-sys/patch-applied exists
- use a single build directory for `cargo build` & `cargo check`

This patch introduces two tests:

1. `couple.test_failover` reproduces heatbeat timeout on a follower
   that leads to a new election.

2. `threesome.test_leader_dispuption` simulates disconnected follower.
   It shouldn't disrupt the leader in this case.

Leader death is already tested in `threesome.test_log_rollback`.

Close https://gitlab.com/picodata/picodata/picodata/-/issues/25

Also, fix the code to pass the test:

1. Use `replace` instead of `insert` when appending entries. They may be
   overridden.
2. Increase pool worker queue size. Raft node may send several messages
   within a single tick. Without this change the second message is
   dropped.

The second change is a poorly designed workaround. Constant limit
doesn't seem to be both cost-effective and reliable at the same time,
but we don't have a better solution at the moment.

Close https://gitlab.com/picodata/picodata/picodata/-/issues/26

It makes testing easier. Monkey patching the handler isn't necessary
anymore. Instead, it's mocked now.

Follower makes a proposal. It's applied on both instances.

This patch also introduces a new API `picolib.raft_status`.

Close https://gitlab.com/picodata/picodata/picodata/-/issues/24

+ tarantool-sys submodule
+ tarantool-patches directory
+ build.rs build script to patch and build tarantool
+ static linking with tarantool
+ refactoring for cli arguments

Close https://gitlab.com/picodata/picodata/picodata/-/issues/23

This patch allows two nodes communicate over `net.box`. It implements C
stored procedure `raft_interact` that is called on the server side.

Close https://gitlab.com/picodata/picodata/picodata/-/issues/17

This patch introduces `traft::ConnectionPool` for async nodes
communication.

Close https://gitlab.com/picodata/picodata/picodata/-/issues/18

Now it's possible to set up initial topology via command-line arguments.
Start one of several nodes (for example, two):

```bash
picodata run --peer 127.0.0.1:3301 --peer 127.0.0.1:3302
```

Single-instance configuration is started as before:

```bash
picodata run
```