Revisit Application.handler flow

[IanMitchell]

It's convoluted, and it isn't clear that it needs to be. There was a lot of thrash during the Request/Response rewrite that might have shook out in a way where we can clean it up. It currently works, but is ugly.

• Can we drop the return new Promise code and move to full async?
• Do we need respond as a callback?

I suspect now that we're threading through Request and not FetchEvent, we can find some improvements