[3pt] Provide box.info.synchro interface

[TarantoolBot]

root document: https://www.tarantool.io/ru/doc/latest/reference/reference_lua/box_info/
since: 2.8.1

Details:
The box.info.synchro leaf provides information about details of
synchronous replication.

In particular quorum represent the current value of synchronous
replication quorum defined by replication_synchro_quorum
configuration parameter because it can be set as dynamic formula
such as N/2+1 and the value depends on current number of replicas.

Since synchronous replication does not commit data immediately
but waits for its propagation to replicas the data sits in a queue
gathering commit responses from remote nodes. Current number of
entries waiting in the queue is shown via queue.len member.

A typical output is the following

tarantool> box.info.synchro
---
- queue:
    len: 0
  quorum: 1
...

The len member shows current number of entries in the queue.
And the quorum member shows an evaluated value of
replication_synchro_quorum parameter.
Requested by @cyrillos in tarantool/tarantool@bce3b58.

• add examples of non-empty state (len > 0, etc.)

Follow-up issue: #2601