Stasis: Fix StasisEnd message ordering

This change corrects message ordering in cases where a channel-related
message can be received after a Stasis/ARI application has received the
StasisEnd message. The StasisEnd message was being passed to
applications directly without waiting for the channel topic to empty.

As a result of this fix, other bugs were also identified and fixed:
* StasisStart messages were also being sent directly to apps and are
  now routed through the stasis message bus properly
* Masquerade monitor datastores were being removed at the incorrect
  time in some cases and were causing StasisEnd messages to not be sent
* General refactoring where necessary for the above
* Unsubscription on StasisEnd timing changes to prevent additional
  messages from following the StasisEnd when they shouldn't

A channel sanitization function pointer was added to reduce processing
and AO2 lookups.

Review: https://reviewboard.asterisk.org/r/4163/
ASTERISK-24501 #close
Reported by: Matt Jordan


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@427788 f38db490-d61c-443f-a65b-d21fe96a405b