fix: tiering statistics handling a bug in clone() implementation (#18342)

Tiering statistics have been broken for some time now, a regression
was introduced in 6f2406b0b6

Bonus fixes an issue where the objects are not assumed to be
of the 'STANDARD' storage-class for the objects that have
not yet tiered, this should be conditional based on the object's
metadata not a default assumption.

This PR also does some cleanup in terms of implementation,

fixes #18070

cmd/erasure.go

@@ -433,6 +433,7 @@ func (er erasureObjects) nsScanner(ctx context.Context, buckets []BucketInfo, wa
 				}
 				logger.LogOnceIf(ctx, cache.save(ctx, er, dataUsageCacheName), "nsscanner-cache-update")
 				updates <- cache.clone()
+
 				lastSave = cache.Info.LastUpdate
 			case v, ok := <-bucketResults:
 				if !ok {
@@ -440,7 +441,7 @@ func (er erasureObjects) nsScanner(ctx context.Context, buckets []BucketInfo, wa
 					cache.Info.NextCycle = wantCycle
 					cache.Info.LastUpdate = time.Now()
 					logger.LogOnceIf(ctx, cache.save(ctx, er, dataUsageCacheName), "nsscanner-channel-closed")
-					updates <- cache
+					updates <- cache.clone()
 					return
 				}
 				cache.replace(v.Name, v.Parent, v.Entry)