applier: drop lag to zero on reconnect

Before the patch if the applier was reconnected, the master would
see downstream lag equal to the time since it replicated the last
txn to this applier.

This happened because applier between reconnects kept the txn
timestamp used for acks. On the master's side the relay was
recreated, received the ack, thought the applier just applied this
txn, and displayed this as a lag.

The test makes a master restart because this is the easiest way to
reproduce it. Most importantly, the applier shouldn't be
re-created, and relay should restart.

Part of #9748

NO_DOC=bugfix
NO_CHANGELOG=later