MB-7873 Ensure blocked indexer tasks die if updater dies

If the updater process dies (shutdown by view group for e.g.),
ensure the corresponding blocked indexer active task will die
as well. This is ensured by keeping both processes linked all
the time.

Change-Id: If326a001c3baa106ea0f2dd46717b881e794fd3b
Reviewed-on: http://review.couchbase.org/25018
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>