For all upcoming and already-processed deferred fetches, we now
implement a quota. We don't support policy headers yet to configure the
quota, hence we use the default quota. That's why we have some tests
failing again.
It involves computing which document to compute the quota for, as well
as computing total length of requests. Additionally, the DevTools
computation for headers now uses the same logic for headers as deferred
fetches.
Signed-off-by: Tim van der Lippe <tvanderlippe@gmail.com>