Address unload order issues for res_stasis* modules

I've noticed when doing a graceful shutdown that the res_stasis_http.so
module gets unloaded before the modules that use it, which causes some
asserts during their unload.

While r386928 was a quick hack to get it to not assert and die, this
patch increases the use counts on res_stasis.so and res_stasis_http.so
properly. It's a bigger change than I expected, hence the review instead
of just committing it.

Review: https://reviewboard.asterisk.org/r/2489/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@388350 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/stasis_app.h

@@ -44,6 +44,10 @@
  * Finally, Stasis apps control channels through the use of the \ref
  * stasis_app_control object, and the family of \c stasis_app_control_*
  * functions.
+ *
+ * Since module unload order is based on reference counting, any module that
+ * uses the API defined in this file must call stasis_app_ref() when loaded,
+ * and stasis_app_unref() when unloaded.
  */
 
 #include "asterisk/channel.h"
@@ -171,6 +175,20 @@ int stasis_app_control_answer(struct stasis_app_control *control);
  */
 struct ast_json *ast_channel_snapshot_to_json(const struct ast_channel_snapshot *snapshot);
 
+/*!
+ * \brief Increment the res_stasis reference count.
+ *
+ * This ensures graceful shutdown happens in the proper order.
+ */
+void stasis_app_ref(void);
+
+/*!
+ * \brief Decrement the res_stasis reference count.
+ *
+ * This ensures graceful shutdown happens in the proper order.
+ */
+void stasis_app_unref(void);
+
 /*! @} */
 
 #endif /* _ASTERISK_STASIS_APP_H */