MB-7535 Start updater if there are update listeners

If there are update listeners waiting for indexing of
specific partitions up to certain sequences, trigger the
updater.

Change-Id: Iac5e96176de1e549372ae58c654e631fe8079155
Reviewed-on: http://review.couchbase.org/24031
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>