qsync: provide box.info.synchro interface for monitoring

In commit 14fa5fd8 (cfg: support symbolic evaluation of
replication_synchro_quorum) we implemented support of
symbolic evaluation of `replication_synchro_quorum` parameter
and there is no easy way to obtain it current run-time value,
ie evaluated number value.

Moreover we would like to fetch queue length on transaction
limbo for tests and extend this statistics in future. Thus
lets add them.

Closes #5191

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

@TarantoolBot document
Title: Provide `box.info.synchro` interface

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

``` Lua
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.