Summary:
Catch up with Posix features
  NewWritableRWFile must fail when file does not exists
  Implement Env::Truncate()
  Adjust Env options optimization functions
  Implement MemoryMappedBuffer on Windows.
Closes https://github.com/facebook/rocksdb/pull/3857

Differential Revision: D8053610

Pulled By: ajkr

fbshipit-source-id: ccd0d46c29648a9f6f496873bc1c9d6c5547487e