Also remove the unnecessary `start_new_session` flag from explel
function.

Raft-rs interface requires the application to process special structure,
the so-called "ready state". It also makes a demand on the processing
workflow. The fields must be processed in a certain sequence, see
<https://docs.rs/raft/0.6.0/raft/index.html#processing-the-ready-state>.

For example, it was a mistake to send `persisted_messages` inside a
transaction, because it's forbidded to do so before persisting the hard
state, but the actual write happens only in the end of transaction.

As for handling `soft_state` and `read_states`, they're useless in a
transaction as they don't persist anything.

The documentation is quite complicated, so this code might be revised
once again later.

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.