MB-11290 Handle stale compactor notification in updater

During certain index group state changes we do not stop the existing
updator running process, but compactor might be stopped and restarted
after state changes are applied. This results in multiple compactor
notifications sent to the updater driver process. The updater driver
expects that only one compactor notification can happen at a time.
But that assumption is incorrect if compactor is stopped and restarted
while updater keeps running. When multiple compactor notification arrive
due to restarts of compactor processes, it needs to send ack to the most
recent process and ignore any further notifications.

Change-Id: Icfab19904057172b31f5c7d6294399bb596be6fb
Reviewed-on: http://review.couchbase.org/37904
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: buildbot <build@couchbase.com>