Fix a crash as a result of propagating MWI or device state over XMPP when the...

Fix a crash as a result of propagating MWI or device state over XMPP when the client is disconnected.

The MWI and device state propagation code wrongly assumes that an XMPP client connection will remain established at all times. This fix corrects that by making the lifetime of the subscription the same as the lifetime of the connection itself. As the connection is established and disconnected the subscription itself is created and destroyed.

(closes issue ASTERISK-18078)
Reported by: elguero


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370152 65c4cc65-6c06-0410-ace0-fbb531ad65f3