Add pubsub unsubscription support so subscriptions do not linger for MWI and device...

Add pubsub unsubscription support so subscriptions do not linger for MWI and device state progatation.

The pubsub code did not attempt to remove subscriptions at all. This has now changed so that if a client is being disconnected it will unsubscribe. It will also unsubscribe at connection time so if it unexpectedly disconnected duplicate subscriptions will not occur.

(closes issue ASTERISK-19882)
Reported by: mattvryan

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370157 65c4cc65-6c06-0410-ace0-fbb531ad65f3