Proper fsync on Windows

[untitaker]

See #6. This blogpost covers this issue in great detail. For Windows, this is mentioned:

On Windows, using FlushFileBuffers() is probably the way to go.

Not really a confident statement, more research needed.

[untitaker]

We currently use _commit, which doesn't try to flush disk buffers. However, there is no indication that FlushFileBuffers is different in that regard.

[Dobatymo]

First: _commit calls FlushFileBuffers internally (according to https://stackoverflow.com/a/1947818). Second: Where do you actually call _commit?

[untitaker]

os.fsync is documented as calling _commit. I guess then this issue is not actionable