Improve async handling

Don't use special callback in node to receive the results. Instead,
use a generic result callback to receive the result. This makes things
a bit more symetric and generic again because then you can choose how
to match the result to the request and you have a generic way to handle
both the sync and async case. We can then also remove the wait method.
This also makes the remote interface and spa interface to objects very
similar.

Make a helper object to receive and dispatch results. Use this in the
helper for enum_params.

Make device use the same result callbacks.

src/pipewire/node.h

@@ -83,6 +83,9 @@ struct pw_node_events {
 	void (*state_changed) (void *data, enum pw_node_state old,
 			       enum pw_node_state state, const char *error);
 
+        /** a result was received */
+	void (*result) (void *data, int seq, int res, const void *result);
+
         /** an event is emited */
 	void (*event) (void *data, const struct spa_event *event);