Directory seeder

[MaddieM4]

The idea is not that complicated, although implementation probably will be: use a directory full of document caches to start up a bunch of clients in parallel, each one serving for a different URL. The document cache files are named with the base64 encoding of the URL.

The idea is that this easily allows a user to host a tree of DJDNS pages, cache them to disk, etc. This could be very useful for regression-testing multipage support.