Don't crash on apply_vbucket_state_reply from stale worker.

When rebalance stops and we terminate the apply_vbucket_state_worker,
the latter might have already sent us some reply. This would cause a
badmatch on queue:out and hence janitor_agent would crash. We fix it by
including worker pid in a reply message. So we can just ignore old
replies.

Seen in MB-11890.

Change-Id: I5eb8fc13fa94179229fad4381059dd77247f4ee3
Reviewed-on: http://review.couchbase.org/40387
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>