introducing janitor_agent_sup

supervisor for janitor agent and and all the processes that need to be
terminated if janitor agent gets killed

Change-Id: I88db72cdf41fa2a39c3e6828e582022643a08422
Reviewed-on: http://review.couchbase.org/38585
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>