replication: add strict ordering for appliers operating in a full mesh

In some cases when an applier processing yielded, other applier might
start some conflicting operation and break replication and database
consistency.
Now applier locks a per-server-id latch before processing a transaction.
This guarantees that there is only one applier request for each server
in progress at each given moment.

The problem was very rare until full mesh topologies in vinyl
became a commonplace.

Fixes gh-3339