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/include/storage/shm_mq.h

@@ -65,6 +65,9 @@ extern void shm_mq_set_handle(shm_mq_handle *, BackgroundWorkerHandle *);
 /* Break connection. */
 extern void shm_mq_detach(shm_mq *);
 
+/* Get the shm_mq from handle. */
+extern shm_mq *shm_mq_get_queue(shm_mq_handle *mqh);
+
 /* Send or receive messages. */
 extern shm_mq_result shm_mq_send(shm_mq_handle *mqh,
 			Size nbytes, const void *data, bool nowait);