Convert presence state events to Stasis-core messages and remove
redundant serializers where possible.

Review: https://reviewboard.asterisk.org/r/2410/


(closes issue ASTERISK-21102)
Patch-by: Kinsey Moore <kmoore@digium.com>


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@385862 65c4cc65-6c06-0410-ace0-fbb531ad65f3