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
........

Merged revisions 378917 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@378918 65c4cc65-6c06-0410-ace0-fbb531ad65f3