Abortable fetch

This makes the following changes:

* Integrates DOM's `AbortSignal`.
* Specifies abort points and how to handle termination.
* Replaces termination reason with an aborted flag.

Tests: web-platform-tests/wpt#6484.

Service worker integration follow-up: w3c/ServiceWorker#1178.

Fixes #447. Closes #563.

Author: jakearchibald