stasis.c: Misc code cleanups.

* Remove some unnecessary RAII_VAR() usage.

* Made the struct stasis_subscription ao2 object use the ao2 lock instead
of a redundant join_lock in the struct for ast_cond_wait().

* Removed locks on some ao2 objects that don't need the lock.

* Made the topic pool entries container use the ao2 template functions.

* Add some missing allocation failure checks.

* Add missing cleanup in off nominal path of dispatch_message().
........

Merged revisions 409270 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@409271 65c4cc65-6c06-0410-ace0-fbb531ad65f3