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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.

Defaults that are more suitable to real-world site archiving.

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).

The native Go implementation of LevelDB.