stream: Guarantee ordering of 'finish' event

In synchronous Writable streams (where the _write cb is called on the
current tick), the 'finish' event (and thus the end() callback) can in
some cases be called before all the write() callbacks are called.

Use a counter, and have stream.Transform rely on the 'prefinish' event
instead of the 'finish' event.

This has zero effect on most streams, but it corrects an edge case and
makes it perform more deterministically, which is a Good Thing.
stream: Handle multi-corking properly

This adds proper support for the following situation:

    w.cork();
    w.write(...);
    w.cork();
    w.write(...);
    w.uncork();
    w.write(...);
    w.uncork();

This is relevant when you have a function (as we do in HTTP) that wants
to use cork, but in some cases, want to have a cork/uncork *around*
that function, without losing the benefits of writev.
http: Use writev instead of the hacky hot end