box: update synchro quorum in on_commit trigger instead of on_replace

Currently, we update the synchronous replication quorum from the
`on_replace` trigger of the `_cluster` space when registering a new
replica. However, during the join process, the replica cannot ack its own
insertion into the `_cluster` space. In the scope of #9723, we are going to
enable synchronous replication for most of the system spaces, including the
`_cluster` space. There are several problems with this:

1. Joining a replica to a 1-member cluster without manual changing of
quorum won't work: it is impossible to commit the insertion into the
`_cluster` space with only 1 node, since the quorum will equal to 2 right
after the insertion.

2. Joining a replica to a 3-member cluster may fail: the quorum will become
equal to 3 right after the insertion, the newly joined replica cannot ACK
its own insertion into the `_cluster` space — if one out of original 3
nodes fails, then reconfiguration will fail.

Generally speaking, it will be impossible to join a new replica to the
cluster, if a quorum, which includes the newly added replica (which cannot
ACK), cannot be gathered.

To solve these problems, let's update the quorum in the `on_commit`
trigger. This way we’ll be able to insert a node regardless of the current
configuration. This somewhat contradicts with the Raft specification, which
requires application of all configuration changes in the `on_replace`
trigger (i.e., as soon as they are persisted in the WAL, without quorum
confirmation), but still forbids several reconfigurations at the same time.

Closes #10087

NO_DOC=<no special documentation page devoted to cluster reconfiguration>