fix: deleteVersions causing xl.meta to have empty Versions[] slice (#14483)

This is a side-affect of the optimization done in PR #13544 which
causes a certain type of delete operations on given object versions
can cause lastVersion indication to be skipped, which leads to
an `xl.meta` where Versions[] slice is empty while the entire
file is intact by itself.

This PR tries to ensure that such files are visible and deletable
by regular means of listing as null 'delete-marker' and also
avoid the situation where this potential issue might arise.

cmd/erasure-healing-common.go

@@ -83,7 +83,10 @@ func commonTime(modTimes []time.Time) (modTime time.Time) {
 }
 
 // Beginning of unix time is treated as sentinel value here.
-var timeSentinel = time.Unix(0, 0).UTC()
+var (
+	timeSentinel     = time.Unix(0, 0).UTC()
+	timeSentinel1970 = time.Unix(0, 1).UTC() // 1970 used for special cases when xlmeta.version == 0
+)
 
 // Boot modTimes up to disk count, setting the value to time sentinel.
 func bootModtimes(diskCount int) []time.Time {