manager: Protect data structures during shutdown.

Occasionally, the manager module would get an "INTERNAL_OBJ: bad magic
number" error on a "core restart gracefully" command if an AMI connection
is established.

* Added ao2_global_obj protection to the sessions global container.

* Fixed the order of unreferencing a session object in session_destroy().

* Removed unnecessary container traversals of the white/black filters
during session_destructor().

(closes issue AST-1242)
Reported by: Guenther Kelleter

Review: https://reviewboard.asterisk.org/r/3144/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@406341 65c4cc65-6c06-0410-ace0-fbb531ad65f3