Set the write location in the pg_receivexlog status messages

This makes it possible for the master to track how much data has
actually been written my pg_receivexlog - and not just how much
has been sent towards it.

src/bin/pg_basebackup/receivelog.c

@@ -359,7 +359,7 @@ ReceiveXlogStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline, char *sysi
 			char		replybuf[sizeof(StandbyReplyMessage) + 1];
 			StandbyReplyMessage *replymsg = (StandbyReplyMessage *) (replybuf + 1);
 
-			replymsg->write = InvalidXLogRecPtr;
+			replymsg->write = blockpos;
 			replymsg->flush = InvalidXLogRecPtr;
 			replymsg->apply = InvalidXLogRecPtr;
 			replymsg->sendTime = now;