Use GetCurrentTimestamp instead of instr_time in GlobalSnapshotGenerate.

CLOCK_MONOTONIC is not ok because it is time since server start, which means
values between nodes are totally uncorrelated. So resort to use usual real time
clock GetCurrentTimestamp (gettimeofday); we should now ensure monotonicity on
our own, but this was done previously anyway.

Ideally, we should use clock_gettime with CLOCK_REALTIME which might provide
nanosecond resolution.

Author: arssher

src/backend/access/transam/global_snapshot.c

@@ -368,23 +368,22 @@ GlobalSnapshotToXmin(GlobalCSN snapshot_global_csn)
  * GlobalSnapshotGenerate
  *
  * Generate GlobalCSN which is actually a local time. Also we are forcing
- * this time to be always increasing. Since now it is not uncommon to have
- * millions of read transactions per second we are trying to use nanoseconds
+ * this time to be always increasing.
+ * XXX since now it is not uncommon to have
+ * millions of read transactions per second should use nanoseconds
  * if such time resolution is available.
  */
 GlobalCSN
 GlobalSnapshotGenerate(bool locked)
 {
-	instr_time	current_time;
 	GlobalCSN	global_csn;
 
 	Assert(track_global_snapshots || global_snapshot_defer_time > 0);
 
 	/*
 	 * TODO: create some macro that add small random shift to current time.
 	 */
-	INSTR_TIME_SET_CURRENT(current_time);
-	global_csn = (GlobalCSN) INSTR_TIME_GET_NANOSEC(current_time);
+	global_csn = GetCurrentTimestamp() * 1000; /* nsec per usec */
 
 	/* TODO: change to atomics? */
 	if (!locked)