vclock: introduce `vclock_nth_element` and `vclock_count_ge`

Two new vclock methods have been added: `vclock_nth_element` and
`vclock_count_ge`.
* `vclock_nth_element` takes n and returns whatever element would occur in
nth position if vclock were sorted. This method is very useful for
synchronous replication because it can be used to find out the lsn of the
last confirmed transaction - it's simply the result of calling this
method with argument {vclock_size - replication_synchro_quorum} (provided
that vclock_size >= replication synchro quorum, otherwise it is obvious
that no transaction has yet been confirmed).
* `vclock_count_ge` takes lsn and returns the number of components whose
value is greater than or equal to lsn. This can be useful to understand
how many replicas have already received a transaction with a given lsn.

Part of #9917

NO_CHANGELOG=Will be added in another commit
NO_DOC=internal

(cherry picked from commit 58f3c93b660499e85f08a4f63373040bcae28732)