This tries to conform with:

https://wiki.debian.org/DebianRepository/UseThirdParty

We also change keyservers to avoid signature flooding
attacks. Ideally, the repository would ship its own key, to avoid
writing that key in the user's keyring, but this fixes the worst of
the problems (short key ID attacks and flooding).

This fixes an awesome bug where restoring from a pipe would not work,
because that's the case where you're going to see short Read()s.

Should help improve the quality of matches.

The idea is to support a set of metadata checks of various
kinds, to select books in large databases for further processing.