Improve replication connection timeouts.

Rename replication_timeout to wal_sender_timeout, and add a new setting
called wal_receiver_timeout that does the same at the walreceiver side.
There was previously no timeout in walreceiver, so if the network went down,
for example, the walreceiver could take a long time to notice that the
connection was lost. Now with the two settings, both sides of a replication
connection will detect a broken connection similarly.

It is no longer necessary to manually set wal_receiver_status_interval to
a value smaller than the timeout. Both wal sender and receiver now
automatically send a "ping" message if more than 1/2 of the configured
timeout has elapsed, and it hasn't received any messages from the other end.

Amit Kapila, heavily edited by me.

src/include/replication/walprotocol.h

@@ -27,6 +27,12 @@ typedef struct
 
 	/* Sender's system clock at the time of transmission */
 	TimestampTz sendTime;
+
+	/*
+	 * If replyRequested is set, the client should reply immediately to this
+	 * message, to avoid a timeout disconnect.
+	 */
+	bool		replyRequested;
 } WalSndrMessage;
 
 
@@ -80,6 +86,12 @@ typedef struct
 
 	/* Sender's system clock at the time of transmission */
 	TimestampTz sendTime;
+
+	/*
+	 * If replyRequested is set, the server should reply immediately to this
+	 * message, to avoid a timeout disconnect.
+	 */
+	bool		replyRequested;
 } StandbyReplyMessage;
 
 /*

src/include/replication/walreceiver.h

@@ -17,7 +17,9 @@
 #include "storage/spin.h"
 #include "pgtime.h"
 
+/* user-settable parameters */
 extern int	wal_receiver_status_interval;
+extern int	wal_receiver_timeout;
 extern bool hot_standby_feedback;
 
 /*

src/include/replication/walsender.h

@@ -24,7 +24,7 @@ extern bool wake_wal_senders;
 
 /* user-settable parameters */
 extern int	max_wal_senders;
-extern int	replication_timeout;
+extern int	wal_sender_timeout;
 
 extern void InitWalSender(void);
 extern void exec_replication_command(const char *query_string);

src/include/replication/walsender_private.h

@@ -37,7 +37,6 @@ typedef struct WalSnd
 	XLogRecPtr	sentPtr;		/* WAL has been sent up to this point */
 	bool		needreload;		/* does currently-open file need to be
 								 * reloaded? */
-	bool		sendKeepalive;	/* do we send keepalives on this connection? */
 
 	/*
 	 * The xlog locations that have been written, flushed, and applied by