Reduce log level for background worker events from LOG to DEBUG1.

Per discussion, LOG is just too chatty for something that will happen
as routinely as this.

Pavel Stehule

src/backend/postmaster/bgworker.c

@@ -341,7 +341,7 @@ BackgroundWorkerStateChange(void)
 		rw->rw_terminate = false;
 
 		/* Log it! */
-		ereport(LOG,
+		ereport(DEBUG1,
 				(errmsg("registering background worker \"%s\"",
 						rw->rw_worker.bgw_name)));
 
@@ -370,7 +370,7 @@ ForgetBackgroundWorker(slist_mutable_iter *cur)
 	slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
 	slot->in_use = false;
 
-	ereport(LOG,
+	ereport(DEBUG1,
 			(errmsg("unregistering background worker \"%s\"",
 					rw->rw_worker.bgw_name)));
 
@@ -741,7 +741,7 @@ RegisterBackgroundWorker(BackgroundWorker *worker)
 	static int	numworkers = 0;
 
 	if (!IsUnderPostmaster)
-		ereport(LOG,
+		ereport(DEBUG1,
 		 (errmsg("registering background worker \"%s\"", worker->bgw_name)));
 
 	if (!process_shared_preload_libraries_in_progress)

src/backend/postmaster/postmaster.c

@@ -2961,7 +2961,8 @@ CleanupBackgroundWorker(int pid,
 		rw->rw_child_slot = 0;
 		ReportBackgroundWorkerPID(rw);	/* report child death */
 
-		LogChildExit(LOG, namebuf, pid, exitstatus);
+		LogChildExit(EXIT_STATUS_0(exitstatus) ? DEBUG1 : LOG,
+							    namebuf, pid, exitstatus);
 
 		return true;
 	}
@@ -5417,7 +5418,7 @@ do_start_bgworker(RegisteredBgWorker *rw)
 {
 	pid_t		worker_pid;
 
-	ereport(LOG,
+	ereport(DEBUG1,
 			(errmsg("starting background worker process \"%s\"",
 					rw->rw_worker.bgw_name)));