relay: update lag on any acked txn

Not only for own txns, but also on the txns authored by other
instances.

Note that the lag isn't updated when the replica got new txns from
another master. The lag still only reflects the replication
between this relay and its specific applier.

The motivation is that otherwise the lag sometimes shows
irrelevant things, like that the replica is very outdated, while
it keeps replicating just fine. Only not txns of this specific
master, who might even turned into a replica itself already.

Closes #9748

NO_DOC=bugfix