In order to do this we have to plumb it through the queue and the
Handler interface, but it should allow fetches of the resources
associated with a page via the IncludeRelatedScope even if it's behind
a redirect.

Introduce an interface to decouple the Enqueue functionality from the
Crawler implementation.

Makes it possible to retry requests for temporary HTTP errors (429,
500, etc).

This change allows more complex scope boundaries, including loosening
edges a bit to include related resources of HTML pages (which makes
for more complete archives if desired).