Enable REF_DEBUG for ast_module_ref / ast_module_unref.

Add ast_module_shutdown_ref for use by modules that can
only be unloaded during graceful shutdown.

When REF_DEBUG is enabled:
* Add an empty ao2 object to struct ast_module.
* Allocate ao2 object when the module is loaded.
* Perform an ao2_ref in each place where mod->usecount is manipulated.
* ao2_cleanup on module unload.

ASTERISK-24479 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/4141/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@431662 65c4cc65-6c06-0410-ace0-fbb531ad65f3