Improvement: Save DiskFile Directly into Response When Cold Cache Miss about turtle

[YukunJ]

On second thought, this is not so straight-forward to change.

The reason why it is done in such a way at first place is that, You don't always succeed in inserting into the Cache. File might be too big to fit into the cache, but you need to load it from disk and serve it to client request anyway.

The HTTP::Response class is probably too coupled in design that in Serialize method it automatically decide if to load the file from disk. This class should be refactored so that the action of Appending file content to the end of the response buffer could be done separately.

Aim to fix this in March during Spring Break.