revert: remove access-frequency ILM tiering (#60)

Reverse the first-parent diff of a3df317ae0,
including the feature branch compatibility and mover follow-up fixes.
Retain the independent multi-pool correctness fixes from #178 and migrate
their shared test fixture away from access-tier code.

Tolerate retired ILM keys and XML, read old v9 statistics while writing v8,
and document migration without moving objects or rewriting their metadata.
Include regression coverage using a historical scanner/writer v9 fixture.

Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/xl-storage.go

@@ -583,7 +583,6 @@ func (s *xlStorage) NSScanner(ctx context.Context, cache dataUsageCache, updates
 	}
 
 	poolIdx, setIdx, _ := s.GetDiskLoc()
-	hotPool, hotPoolOK := globalILMConfig.accessCfg().HotPool()
 
 	disks, err := objAPI.GetDisks(poolIdx, setIdx)
 	if err != nil {
@@ -593,7 +592,6 @@ func (s *xlStorage) NSScanner(ctx context.Context, cache dataUsageCache, updates
 	cache.Info.updates = updates
 
 	dataUsageInfo, err := scanDataFolder(ctx, disks, s, cache, func(item scannerItem) (sizeSummary, error) {
-		item.poolIdx = poolIdx
 		// Look for `xl.meta/xl.json' at the leaf.
 		if !strings.HasSuffix(item.Path, SlashSeparator+xlStorageFormatFile) &&
 			!strings.HasSuffix(item.Path, SlashSeparator+xlStorageFormatFileV1) {
@@ -657,9 +655,6 @@ func (s *xlStorage) NSScanner(ctx context.Context, cache dataUsageCache, updates
 				sizeS.versions++
 			}
 			sizeS.totalSize += sz
-			if hotPoolOK && poolIdx == hotPool {
-				sizeS.hotTierSize += sz
-			}
 
 			// Skip tier accounting if object version is a delete-marker or a free-version
 			// tracking deleted transitioned objects