Hot Standby feedback for avoidance of cleanup conflicts on standby.

Standby optionally sends back information about oldestXmin of queries
which is then checked and applied to the WALSender's proc->xmin.
GetOldestXmin() is modified slightly to agree with GetSnapshotData(),
so that all backends on primary include WALSender within their snapshots.
Note this does nothing to change the snapshot xmin on either master or
standby. Feedback piggybacks on the standby reply message.
vacuum_defer_cleanup_age is no longer used on standby, though parameter
still exists on primary, since some use cases still exist.

Simon Riggs, review comments from Fujii Masao, Heikki Linnakangas, Robert Haas

src/include/replication/walprotocol.h

@@ -56,6 +56,15 @@ typedef struct
 	XLogRecPtr	flush;
 	XLogRecPtr	apply;
 
+	/*
+	 * The current xmin and epoch from the standby, for Hot Standby feedback.
+	 * This may be invalid if the standby-side does not support feedback,
+	 * or Hot Standby is not yet available.
+	 */
+	TransactionId	xmin;
+	uint32			epoch;
+
+
 	/* Sender's system clock at the time of transmission */
 	TimestampTz sendTime;
 } StandbyReplyMessage;

src/include/replication/walreceiver.h

@@ -18,6 +18,7 @@
 
 extern bool am_walreceiver;
 extern int wal_receiver_status_interval;
+extern bool hot_standby_feedback;
 
 /*
  * MAXCONNINFO: maximum size of a connection string.