Cascading replication feature for streaming log-based replication.

Standby servers can now have WALSender processes, which can work with
either WALReceiver or archive_commands to pass data. Fully updated
docs, including new conceptual terms of sending server, upstream and
downstream servers. WALSenders terminated when promote to master.

Fujii Masao, review, rework and doc rewrite by Simon Riggs

src/backend/replication/syncrep.c

@@ -469,6 +469,13 @@ SyncRepGetStandbyPriority(void)
 	int			priority = 0;
 	bool		found = false;
 
+	/*
+	 * Since synchronous cascade replication is not allowed, we always
+	 * set the priority of cascading walsender to zero.
+	 */
+	if (am_cascading_walsender)
+		return 0;
+
 	/* Need a modifiable copy of string */
 	rawstring = pstrdup(SyncRepStandbyNames);