1. Encapsulate topology access in a dedicated function.
2. Remove obsolete CachedCell

`PoolWorker` is majorly reworked and now supports executing various
requests asynchronously. The caller is notified of the request's outcome
via callback. The connection is dropped on error only if there are no
requests awaiting response (or timeout).

`ConnectionPool` now has
- methods `call_and_wait`/`call_and_wait_timeout` which yield
- method `call` which doesn't yield but accepts a callback

Every field of Peer now has a corresponding unit struct defined in the
`peer_field` module, which implements the `PeerFieldDef` trait which in
turn contains information about the field's name, tarantool type and
rust type.

This information currently is used in the `field_by_raft_id` method
and is generated by the `define_peer_fields` macro, which also
automates the generation of the format (`peer_format` function) for the
"raft_group" space.

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

Exit only after processing `light_ready`. It doesn't affect much the
behavior, but simplifies main_loop decomposition.

`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