res_pjsip res_pjsip_mwi: Misc fixes and cleanups.

res_pjsip res_pjsip_mwi: Misc fixes and cleanups.

* Eliminated RAII_VAR() usage in
ast_sip_persistent_endpoint_update_state().

* Added a missing allocation failure check to
persistent_endpoint_find_or_create().

* Made persistent_endpoint_find_or_create() create the new object without
a lock as it isn't needed.

* Cleaned up some ao2 container allocation idioms.

* Reordered res_pjsip_mwi.c load_module() and unload_module()

Change-Id: If8ce88fbd82a0c72a37a2388f74f77237a6a36a8