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/data-usage-utils.go

@@ -46,8 +46,7 @@ type BucketTargetUsageInfo struct {
 // - total objects in a bucket
 // - object size histogram per bucket
 type BucketUsageInfo struct {
-	Size        uint64 `json:"size"`
-	HotTierSize uint64 `json:"hotTierSize,omitempty"`
+	Size uint64 `json:"size"`
 	// Following five fields suffixed with V1 are here for backward compatibility
 	// Total Size for objects that have not yet been replicated
 	ReplicationPendingSizeV1 uint64 `json:"objectsPendingReplicationTotalSize"`
@@ -79,10 +78,6 @@ type DataUsageInfo struct {
 	// LastUpdate is the timestamp of when the data usage info was last updated.
 	// This does not indicate a full scan.
 	LastUpdate time.Time `json:"lastUpdate"`
-	// ScannerCycle changes only after a complete scanner pass. Background
-	// consumers use it to distinguish a partial cache update from a baseline
-	// that has visited every bucket and server pool.
-	ScannerCycle uint32 `json:"scannerCycle,omitempty"`
 
 	// Objects total count across all buckets
 	ObjectsTotalCount uint64 `json:"objectsCount"`