Fix request and response's body story

* It is now clear what bytes are counted for progress events.
* Is now clear whether content encodings are removed.
* fetch() no longer needs to handle the process response body and
end-of-file callbacks
* concept-header-list-delete is a thing
* An Apache quirk is now documented

This affects:

* https://github.com/slightlyoff/ServiceWorker/issues/339
* https://www.w3.org/Bugs/Public/show_bug.cgi?id=25587
* https://www.w3.org/Bugs/Public/show_bug.cgi?id=23582