Async: Redo HTTPResponseWriter writes to not use DispatchIO

[helje5]

We may be able to avoid a lot of copying when we use a DispatchSource to wait for the socket being ready. And then do the sends manually until EWOULDBLOCK. A buffer is maintained for pipelining anyways.
(it is a little harder than it sounds because we may need to split blocks).

When doing this, we should also prepare for writev in the same run. (and maybe add API which supports that on the user side)