Revert back-branch changes in behavior of age(xid).

Per discussion, it does not seem like a good idea to change the behavior of
age(xid) in a minor release, even though the old definition causes the
function to fail on hot standby slaves.  Therefore, revert commit
5829387381d2e4edf84652bb5a712f6185860670 and follow-on commits in the back
branches only.

src/backend/access/transam/xact.c

@@ -392,30 +392,6 @@ GetCurrentTransactionIdIfAny(void)
 	return CurrentTransactionState->transactionId;
 }
 
-/*
- *	GetStableLatestTransactionId
- *
- * Get the XID once and then return same value for rest of transaction.
- * Acts as a useful reference point for maintenance tasks.
- */
-TransactionId
-GetStableLatestTransactionId(void)
-{
-	static LocalTransactionId lxid = InvalidLocalTransactionId;
-	static TransactionId stablexid = InvalidTransactionId;
-
-	if (lxid != MyProc->lxid)
-	{
-		lxid = MyProc->lxid;
-		stablexid = GetTopTransactionIdIfAny();
-		if (!TransactionIdIsValid(stablexid))
-			stablexid = ReadNewTransactionId();
-	}
-
-	Assert(TransactionIdIsValid(stablexid));
-
-	return stablexid;
-}
 
 /*
  * AssignTransactionId

src/backend/utils/adt/xid.c

@@ -87,13 +87,13 @@ xideq(PG_FUNCTION_ARGS)
 }
 
 /*
- *		xid_age			- compute age of an XID (relative to latest stable xid)
+ *		xid_age			- compute age of an XID (relative to current xact)
  */
 Datum
 xid_age(PG_FUNCTION_ARGS)
 {
 	TransactionId xid = PG_GETARG_TRANSACTIONID(0);
-	TransactionId now = GetStableLatestTransactionId();
+	TransactionId now = GetTopTransactionId();
 
 	/* Permanent XIDs are always infinitely old */
 	if (!TransactionIdIsNormal(xid))

src/include/access/xact.h

@@ -198,7 +198,6 @@ extern TransactionId GetTopTransactionId(void);
 extern TransactionId GetTopTransactionIdIfAny(void);
 extern TransactionId GetCurrentTransactionId(void);
 extern TransactionId GetCurrentTransactionIdIfAny(void);
-extern TransactionId GetStableLatestTransactionId(void);
 extern SubTransactionId GetCurrentSubTransactionId(void);
 extern CommandId GetCurrentCommandId(bool used);
 extern TimestampTz GetCurrentTransactionStartTimestamp(void);