Various simple optimizations for the chunked upload process.

Various simple optimizations for the chunked upload process.

* This adds an UnregisteredLocalFile::setLocalReference()
  function, which is used to avoid an extra GET request.
* This removes the useless rename() in stashFile(). We just
  need to make sure the stashed file has the right extension.
  Getting rid of the rename makes setLocalReference() usable.
* Also adds some debug logging with ellapsed time.

Change-Id: I087701ad0c27a4eba74591e6b49f5667b011424c