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: http://svn.asterisk.org/svn/asterisk/trunk@387630 f38db490-d61c-443f-a65b-d21fe96a405b
Clean up documentation; prevent ref leak on exit

This patch:
 * Cleans up some doxygen
 * Prevents leaking the system level Stasis topics and messages
   on exit (users of valgrind will be happier)


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@387633 f38db490-d61c-443f-a65b-d21fe96a405b