Don't send protocol messages to a shm_mq that no longer exists.

Commit 2bd9e412f9 introduced a mechanism
for relaying protocol messages from a background worker to another
backend via a shm_mq.  However, there was no provision for shutting
down the communication channel.  Therefore, a protocol message sent
late in the shutdown sequence, such as a DEBUG message resulting from
cranking up log_min_messages, could crash the server.  To fix, install
an on_dsm_detach callback that disables sending messages to the shm_mq
when the associated DSM is detached.

src/backend/storage/ipc/shm_mq.c

@@ -745,6 +745,15 @@ shm_mq_detach(shm_mq *mq)
 		SetLatch(&victim->procLatch);
 }
 
+/*
+ * Get the shm_mq from handle.
+ */
+shm_mq *
+shm_mq_get_queue(shm_mq_handle *mqh)
+{
+	return mqh->mqh_queue;
+}
+
 /*
  * Write bytes into a shared message queue.
  */