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/config-current.go

@@ -227,7 +227,7 @@ func initHelp() {
 		},
 		config.HelpKV{
 			Key:         config.ILMSubSys,
-			Description: "manage ILM settings for expiration, transition, and access-tier workers",
+			Description: "manage ILM settings for expiration and transition workers",
 			Optional:    true,
 		},
 	}
@@ -704,9 +704,6 @@ func applyDynamicConfigForSubSys(ctx context.Context, objAPI ObjectLayer, s conf
 			if globalExpiryState != nil {
 				globalExpiryState.ResizeWorkers(ilmCfg.ExpirationWorkers)
 			}
-			if globalAccessTierState != nil {
-				globalAccessTierState.UpdateWorkers(ilmCfg.AccessWorkers)
-			}
 			globalILMConfig.update(ilmCfg)
 		}
 	}