add CopyObject optimization when source and destination are same (#10170)

when source and destination are same and versioning is enabled
on the destination bucket - we do not need to re-create the entire
object once again to optimize on space utilization.

Cases this PR is not supporting

- any pre-existing legacy object will not
  be preserved in this manner, meaning a new
  dataDir will be created.

- key-rotation and storage class changes
  of course will never re-use the dataDir

cmd/object-api-datatypes.go

@@ -198,11 +198,14 @@ type ObjectInfo struct {
 	PutObjReader *PutObjReader  `json:"-"`
 
 	metadataOnly bool
+	versionOnly  bool // adds a new version, only used by CopyObject
 	keyRotation  bool
 
 	// Date and time when the object was last accessed.
 	AccTime time.Time
 
+	Legacy bool // indicates object on disk is in legacy data format
+
 	// backendType indicates which backend filled this structure
 	backendType BackendType
 }