- 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
```

Module `traft::storage` doesn't panic anymore. Instead, it gracefully
returns errors. All unwraps are moved to the calling functions.

Also, the code is supplemented by tests, and debug logs are removed.

1. Merge `on_ready` handler with the `loop_fn` body into
   the larger `raft_main` function.

2. Get rid of debug logs.

1. Prefer using `"str".into()` since it's briefer.
2. Preserve `"str".to_owned()` if type inference can't help.

1. Fix PICOLIB_AUTORUN env var naming.
2. Add an assertion that it works as expected.
3. Show output if listing tests fails.
3. Run inner tests with listen port :0 to avoid
   "address already in use" errors.

It's a part of networking development.

Also, this patch refactors error handling introduced in previous commit
(see 96aba97a). A single struct is replaced with the enum, common for
all `try_from` functions.

1. Replace `From` trait for raft::Entry with the `TryFrom`. Fairly
   handle its errors in the storage.

2. Replace some other unwraps with "expect" with a more clear message.

3. Support all of `raft::EntryType` variants.