box: rework clear_synchro_queue to commit everything

It is possible that a new leader (elected either via raft or manually or
via some user-written election algorithm) loses the data that the old
leader has successfully committed and confirmed.

Imagine such a situation: there are N nodes in a replicaset, the old
leader, denoted A, tries to apply some synchronous transaction. It is
written on the leader itself and N/2 other nodes, one of which is B.
The transaction has thus gathered quorum, N/2 + 1 acks.

Now A writes CONFIRM and commits the transaction, but dies before the
confirmation reaches any of its followers. B is elected the new leader and it
sees that the last A's transaction is present on N/2 nodes, so it doesn't have a
quorum (A was one of the N/2 + 1).

Current `clear_synchro_queue()` implementation makes B roll the transaction
back, leading to rollback after commit, which is unacceptable.

To fix the problem, make `clear_synchro_queue()` wait until all the rows from
the previous leader gather `replication_synchro_quorum` acks.

In case the quorum wasn't achieved during replication_synchro_timeout, rollback
nothing and wait for user's intervention.

Closes #5435

Co-developed-by: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>