cfg: support symbolic evaluation of replication_synchro_quorum

When synchronous replication is used we prefer a user to specify
a quorum number, ie the number of replicas where data must be
replicated before the master node continue accepting new
transactions.

This is not very convenient since a user may not know initially
how many replicas will be used. Moreover the number of replicas
may vary dynamically. For this sake we allow to specify the
number of quorum in a symbolic way.

For example

box.cfg {
	replication_synchro_quorum = "N/2+1",
}

where `N` is a number of registered replicas in a cluster.
Once new replica attached or old one detached the number
is renewed and propagated.

Internally on each replica_set_id() and replica_clear_id(),
ie at moment when replica get registered or unregistered,
we call box_update_replication_synchro_quorum() helper which
finds out if evaluation of replication_synchro_quorum is
needed and if so we calculate new replication_synchro_quorum
value based on number of currently registered replicas. Then
we notify dependent systems such as qsync and raft to update
their guts.

Note: we do *not* change the default settings for this option,
it remains 1 by default for now. Change the default option should
be done as a separate commit once we make sure that everything is
fine.

Closes #5446

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

@TarantoolBot document
Title: Support dynamic evaluation of synchronous replication quorum

Setting `replication_synchro_quorum` option to an explicit integer
value was introduced rather for simplicity sake mostly. For example
if the cluster's size is not a constant value and new replicas are
connected in dynamically then an administrator might need to increase
the option by hands or by some other external tool.

Instead one can use a dynamic evaluation of a quorum value via formal
representation using symbol `N` as a current number of registered replicas
in a cluster.

For example the canonical definition for a quorum (ie majority
of members in a set) of `N` replicas is `N/2+1`. For such configuration
define

```
box.cfg {replication_synchro_quorum = "N/2+1"}
```

The formal statement allows to provide a flexible configuration but keep
in mind that only canonical quorum (and bigger values, say `N` for all
replicas) guarantees data reliability and various weird forms such as
`N/3+1` while allowed may lead to unexpected results.