`KVCell` contains an optional value and provides access by key in two
flavors:

```rust
fn take_or_drop(&mut self, key: &K) -> Option<T>
fn take_or_keep(&mut self, key: &K) -> Option<T>
```

Both return `Some` value only if provided key matches the contained one.

The behavior differs when the provided key doesn't match:
- `take_or_drop` always leaves the cell empty. It supersedes
  `CachedCell` and copies its behavior.
- `take_or_keep` retains contained value. It's a replacement for
  `JointStateLatch`.

Single-out ConfChange generation logics into a separate clean function.

Georgy Moshkin authored 2 years ago and Georgy Moshkin committed 2 years ago

replication_factor now goes into cluster_state

Yaroslav Dynnikov authored 2 years ago and Georgy Moshkin committed 2 years ago

The final goal is to encapsulate access to Storage, but at first we split it
into two parts - one for raft data, and another one for picodata stuff.

This patch introduces the first one - RaftSpaceAccess. It isn't used in
code yet, but already displays the future API.

If makes the code neater.

Tell a story.

Tarantool remote requests are handled in a IProto thread, and
on_shutdown triggers are handled in the TX thread.

During shutdown we send a request to the leader, which introduces a race
condition in case we are the leader. If iproto thread shuts down before
the request is sent, it will never be handled.

The fix is simple: in case we are the leader during shutdown, don't send
the request, instead handle the request directly in the TX thread.

To highlight all the logging messages containing the given key do this:
* rust: `tlog::highlight_key(log_key, Some(color))`.
* lua: `picolib.log.highlight_key(log_key, color)`.

To disable highlight for the given key do this:
* rust: `tlog::highlight_key(log_key, None)`.
* lua: `picolib.log.highlight_key(log_key)`.

To disable highlighting of all keys do this:
* rust: `tlog::clear_highlight()`.
* lua: `picolib.log.clear_highlight()`.