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/api-headers.go

@@ -187,7 +187,7 @@ func setObjectHeaders(w http.ResponseWriter, objInfo ObjectInfo, rs *HTTPRangeSp
 	if objInfo.IsRemote() {
 		// Check if object is being restored. For more information on x-amz-restore header see
 		// https://docs.aws.amazon.com/AmazonS3/latest/API/API_HeadObject.html#API_HeadObject_ResponseSyntax
-		w.Header()[xhttp.AmzStorageClass] = []string{objInfo.TransitionTier}
+		w.Header()[xhttp.AmzStorageClass] = []string{objInfo.TransitionedObject.Tier}
 	}
 
 	if lc, err := globalLifecycleSys.Get(objInfo.Bucket); err == nil {