Renew mvcc monitoring metrics

[alyapunov]

In #8122 one of mvcc objects - memtx_tx_conflict - was removed as a class. However, it was not removed from mvcc memory monitoring by several reasons:

1. It's a breaking change. At least metrics module relies on existence of this metric. Fix of that module and deprecation policy is needed.
2. It is expected that some other objects can be removed in further refactoring and it is reasonable to postpone metrics renewal to that moment.
3. It's a bit complicated and requires simultaneous update of (at least) metrics module.

We should return to this issues once mvcc refactoring is completed.

After #8654 the same story happened to tx_conflict_tracker.