Retain XMPP filters across reconnections so external modules continue to function as expected.

Previously if an XMPP client reconnected any filters added by an external module were lost.
This issue exhibited itself with chan_motif not receiving and reacting to Jingle signaling.

(closes issue ASTERISK-20916)
Reported by: kuj


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@378917 f38db490-d61c-443f-a65b-d21fe96a405b