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.