xl: Rename getOrderedDisks as shuffleDisks appropriately. (#3796)

This PR is for readability cleanup

- getOrderedDisks as shuffleDisks
- getOrderedPartsMetadata as shufflePartsMetadata

Distribution is now a second argument instead being the
primary input argument for brevity.

Also change the usage of type casted int64(0), instead
rely on direct type reference as `var variable int64` everywhere.

cmd/object-handlers.go

@@ -133,7 +133,7 @@ func (api objectAPIHandlers) GetObjectHandler(w http.ResponseWriter, r *http.Req
 	}
 
 	// Get the object.
-	startOffset := int64(0)
+	var startOffset int64
 	length := objInfo.Size
 	if hrange != nil {
 		startOffset = hrange.offsetBegin
@@ -620,7 +620,7 @@ func (api objectAPIHandlers) CopyObjectPartHandler(w http.ResponseWriter, r *htt
 	}
 
 	// Get the object.
-	startOffset := int64(0)
+	var startOffset int64
 	length := objInfo.Size
 	if hrange != nil {
 		startOffset = hrange.offsetBegin