Summary. A change https://reviews.facebook.net/differential/diff/224721/
  Has attempted to move common functionality out of platform dependent
  code to a new facility called file_reader_writer.
  This includes:
  - perf counters
  - Buffering
  - RateLimiting

  However, the change did not attempt to refactor Windows code.
  To mitigate, we introduce new quering interfaces such as UseOSBuffer(),
  GetRequiredBufferAlignment() and ReaderWriterForward()
  for pure forwarding where required.
  Introduce WritableFile got a new method Truncate(). This is to communicate
  to the file as to how much data it has on close.
   - When space is pre-allocated on Linux it is filled with zeros implicitly,
    no such thing exist on Windows so we must truncate file on close.
   - When operating in unbuffered mode the last page is filled with zeros but we still want to truncate.

   Previously, Close() would take care of it but now buffer management is shifted to the wrappers and the file has
   no idea about the file true size.

   This means that Close() on the wrapper level must always include
   Truncate() as well as wrapper __dtor should call Close() and
   against double Close().
   Move buffered/unbuffered write logic to the wrapper.
   Utilize Aligned buffer class.
   Adjust tests and implement Truncate() where necessary.
   Come up with reasonable defaults for new virtual interfaces.
   Forward calls for RandomAccessReadAhead class to avoid double
   buffering and locking (double locking in unbuffered mode on WIndows).