Async: Redo reads to not use DispatchIO

[helje5]

This goes along issue #2 but has a different motivation and less actual performance impact (unless we also add an option to let the user choose what kind of buffer to produce, e.g. Data instead of DispatchData).

By using a DispatchSource to wait for read events, we could probably avoid an own read buffer (and all the logic attached) and immediately suspend reading.