Fix performance for cache misses.

[hgzimmerman]

Actually creating the in-memory file requires a copy. Serializing the file to a request requires another copy. That first copy should be avoided if possible.

So to bring cache misses more or less in parity with normal file reads do the following.

• Get the size using https://doc.rust-lang.org/std/fs/struct.Metadata.html
• Determine if an insertion is required using just the size and the path.
  • This will remove the required space.
• If an insertion is required, then insert and return the sized file.
• If not, get a file handle, similar to the NamedFile in Rocket and return that instead.
• Return an Either<SizedFile,NamedFile> to be used in the CachedFile.

Also rename everything.