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/server_utils_test.go

@@ -91,7 +91,7 @@ func calculateStreamContentLength(dataLen, chunkSize int64) int64 {
 	}
 	chunksCount := int64(dataLen / chunkSize)
 	remainingBytes := int64(dataLen % chunkSize)
-	streamLen := int64(0)
+	var streamLen int64
 	streamLen += chunksCount * calculateSignedChunkLength(chunkSize)
 	if remainingBytes > 0 {
 		streamLen += calculateSignedChunkLength(remainingBytes)