CBQE-950: rest alerts settings implementaion

implement/modify set_alerts_settings, get_alerts_settings, disable_alerts

Change-Id: I36fa43ea56627b57ce56a084d6cb505e2bb7c1d6
Reviewed-on: http://review.couchbase.org/23686
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>