Migrate SHARED's use of the VarSet AMI event to Stasis-Core

This patch removes the direct call to AMI from the SHARED function
and instead call Stasis-Core. Stasis-Core delivers the notification
that a shared variable has changed on a channel to all interested
consumers.

(issue ASTERISK-21462)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@387630 65c4cc65-6c06-0410-ace0-fbb531ad65f3