sql: avoid offsetting merge timestamp to reduce unnecessary waiting

In 1497315 we added an offset to the
merge timestamp to attempt to diagnose an issue we were seeing with
backfills.

That issue ended up being caused by #144650.

The offset makes schema changes wait longer before completing, so since
it turned out not to be necessary, this commit reverts to the old
behavior.

Release note: None

Author: rafiss

pkg/sql/backfill.go

@@ -2994,12 +2994,11 @@ func indexTruncateInTxn(
 // the next resume, a mergeTimestamp newer than the GC time will be picked and
 // the job can continue.
 func getMergeTimestamp(ctx context.Context, clock *hlc.Clock) hlc.Timestamp {
-	// Use the current timestamp plus the maximum allowed offset to account for
-	// potential clock skew across nodes. The chosen timestamp must be greater
-	// than all commit timestamps used so far. This may result in seeing rows
-	// that are already present in the index being merged, but that’s fine as
-	// they will be treated as no-ops.
-	ts := clock.Now().AddDuration(clock.MaxOffset())
+	// Use the current timestamp since by this point we only have one descriptor
+	// version and no one can commit with the old version due to transaction
+	// deadlines from the lease manager. Anything after that version will write to
+	// both the backfilled and temporary index.
+	ts := clock.Now()
 	log.Dev.Infof(ctx, "merging all keys in temporary index before time %v", ts)
 	return ts
 }