pulse: fix drain

The drain operation does not complete with a sync from the server but
with an event from the stream. Set a flag in the operation that it
completes with a sync. Keep all operations without a sync around in
the list.

pipewire-pulseaudio/src/stream.c

@@ -461,6 +461,8 @@ static void stream_drained(void *data)
 {
 	pa_stream *s = data;
 
+	pw_log_debug("drained");
+
 	if (s->drain) {
 		pa_operation *o = s->drain;
 		pa_operation_ref(o);