Avoid extra locks in GetSnapshotData if old_snapshot_threshold < 0

On a big NUMA machine with 1000 connections in saturation load
there was a performance regression due to spinlock contention, for
acquiring values which were never used.  Just fill with dummy
values if we're not going to use them.

This patch has not been benchmarked yet on a big NUMA machine, but
it seems like a good idea on general principle, and it seemed to
prevent an apparent 2.2% regression on a single-socket i7 box
running 200 connections at saturation load.

src/backend/storage/ipc/procarray.c

@@ -1759,14 +1759,26 @@ GetSnapshotData(Snapshot snapshot)
 	snapshot->regd_count = 0;
 	snapshot->copied = false;
 
-	/*
-	 * Capture the current time and WAL stream location in case this snapshot
-	 * becomes old enough to need to fall back on the special "old snapshot"
-	 * logic.
-	 */
-	snapshot->lsn = GetXLogInsertRecPtr();
-	snapshot->whenTaken = GetSnapshotCurrentTimestamp();
-	MaintainOldSnapshotTimeMapping(snapshot->whenTaken, xmin);
+	if (old_snapshot_threshold < 0)
+	{
+		/*
+		 * If not using "snapshot too old" feature, fill related fields with
+		 * dummy values that don't require any locking.
+		 */
+		snapshot->lsn = InvalidXLogRecPtr;
+		snapshot->whenTaken = 0;
+	}
+	else
+	{
+		/*
+		 * Capture the current time and WAL stream location in case this
+		 * snapshot becomes old enough to need to fall back on the special
+		 * "old snapshot" logic.
+		 */
+		snapshot->lsn = GetXLogInsertRecPtr();
+		snapshot->whenTaken = GetSnapshotCurrentTimestamp();
+		MaintainOldSnapshotTimeMapping(snapshot->whenTaken, xmin);
+	}
 
 	return snapshot;
 }