Fix GetNewTransactionId()'s interaction with xidVacLimit.

Commit ad308058 switched to returning a FullTransactionId, but
failed to load the potentially updated value in the case where
xidVacLimit is reached and we release and reacquire the lock.
Repair, closing bug #15727.

While reviewing that commit, also fix the size computation used
by EstimateTransactionStateSize() and switch to the mul_size()
macro traditionally used in such expressions.

Author: Thomas Munro
Reported-by: Roman Zharkov
Discussion: https://postgr.es/m/15727-0be246e7d852d229%40postgresql.org

src/backend/access/transam/varsup.c

@@ -159,7 +159,8 @@ GetNewTransactionId(bool isSubXact)
 
 		/* Re-acquire lock and start over */
 		LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
-		xid = XidFromFullTransactionId(ShmemVariableCache->nextFullXid);
+		full_xid = ShmemVariableCache->nextFullXid;
+		xid = XidFromFullTransactionId(full_xid);
 	}
 
 	/*

src/backend/access/transam/xact.c

@@ -5151,7 +5151,7 @@ EstimateTransactionStateSpace(void)
 		nxids = add_size(nxids, s->nChildXids);
 	}
 
-	return add_size(size, sizeof(SerializedTransactionState) * nxids);
+	return add_size(size, mul_size(sizeof(TransactionId), nxids));
 }
 
 /*