Turn off WalSender keepalives by default, users can enable if desired

src/include/replication/walsender_private.h

@@ -37,6 +37,7 @@ 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