applier: move applier_txn_last_tm into applier

It was stored in struct replica, now is in struct applier. The
motivation is that applier-specific data must be inside the
applier.

Also it makes the next commits look more logical. They are going
to change this timestamp when applier progresses through its state
machine. It looks strange when the applier is changing the replica
object. Replica is on an upper level in the hierarchy. It owns the
applier and the applier ideally mustn't know about struct replica
(hardly possible to achieve), or at least not change it (this is
feasible).

In scope of #9748

NO_DOC=internal
NO_TEST=refactoring
NO_CHANGELOG=refactoring