Support asynchronous file logging

[greaka]

It's a minor thing, really, but it would be nice if file logging wouldn't block until it's fully written to disk. I assume this is by design, so there would probably be the need of a parameter or separate function call to get this right.

[megai2]

How much async it should be? I can simple remove flush, but this can be not enough.

[megai2]

added one without fflush at least