Don't flush files if they weren't written to

[ghost]

Before this PR, files that were opened and then never written to would get flushed on drop, which is a useless operation and especially concerning when small files are read.

Newly opened files now start in flushed state. When constructing async_std::fs::File from a std::fs::File, we still have to start in "dirty" (not flushed) state because we don't know if the handle was previously written to.

[yoshuawuyts]

This looks good!

I'm kind of wondering here if we should have a small internal enum for the flushed state? We're passing bools in params which doesn't make sense when reading. It's spread over multiple files which makes it harder to jump to the definition also. Mostly just a thought, and not sure if worth it.

[skade]

I agree with @yoshuawuyts there, I think an enum would be preferable. Still, as this is internal, I'd merge and patch.

[yoshuawuyts]

Yep, let's merge this!