This allows users of crawl-as-a-library to recover from unexpected
errors as a last resort.

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.

The whole URLInfo structure, while neat, is unused except for the
purpose of verifying if we have already seen a specific URL.

The presence check is also now limited to Enqueue().

The output stage can now write to size-limited, rotating WARC files
using a user-specified pattern, so that output files are always
unique.

Allow users to add to the exclude regexp lists easily.

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

Handler errors are fatal, so that an error writing the WARC output
will cause the crawl to abort.

Detect write errors (both on the database and to the WARC output) and
abort with an error message.

Also fix a bunch of harmless lint warnings.

Since such bandwidth limiting is not provided by crawl directly, tell
users there is another solution. Once/if crawl implements that on its
own, that notice could be removed.