Invent our own prefix values.

We're now dropping all pretense of keeping the database directly
compatible with sup's current xapian backend. (But perhaps someone
might write a new nothmuch backend for sup in the future.)

In coming up with the prefix values here, I tried to follow the
conventions of http://xapian.org/docs/omega/termprefixes.html as
closely as makes sense, (with some domain translation from "web"
to "email archive").