Report WAL flush, not insert, position in replication IDENTIFY_SYSTEM

When beginning streaming replication, the client usually issues the
IDENTIFY_SYSTEM command, which used to return the current WAL insert
position. That's not suitable for the intended purpose of that field,
however. pg_receivexlog uses it to start replication from the reported
point, but if it hasn't been flushed to disk yet, it will fail. Change
IDENTIFY_SYSTEM to report the flush position instead.

Backpatch to 9.1 and above. 9.0 doesn't report any WAL position.

doc/src/sgml/protocol.sgml

@@ -1371,7 +1371,7 @@ The commands accepted in walsender mode are:
       </term>
       <listitem>
       <para>
-       Current xlog write location. Useful to get a known location in the
+       Current xlog flush location. Useful to get a known location in the
        transaction log where streaming can start.
       </para>
       </listitem>

src/backend/replication/walsender.c

@@ -316,7 +316,7 @@ IdentifySystem(void)
 		logptr = GetStandbyFlushRecPtr();
 	}
 	else
-		logptr = GetInsertRecPtr();
+		logptr = GetFlushRecPtr();
 
 	snprintf(tli, sizeof(tli), "%u", ThisTimeLineID);