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).

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.

Now generates well-formed, indexable WARC files.