fix: various performance improvements to tiering (#12965)

- deletes should always Sweep() for tiering at the
  end and does not need an extra getObjectInfo() call
- puts, copy and multipart writes should conditionally
  do getObjectInfo() when tiering targets are configured
- introduce 'TransitionedObject' struct for ease of usage
  and understanding.
- multiple-pools optimization deletes don't need to hold
  read locks verifying objects across namespace and pools.

cmd/tier.go

@@ -115,6 +115,11 @@ func (config *TierConfigMgr) Add(ctx context.Context, tier madmin.TierConfig) er
 	return nil
 }
 
+// Empty returns if tier targets are empty
+func (config *TierConfigMgr) Empty() bool {
+	return len(config.ListTiers()) == 0
+}
+
 // ListTiers lists remote tiers configured in this deployment.
 func (config *TierConfigMgr) ListTiers() []madmin.TierConfig {
 	config.RLock()