Invent our own prefix values.

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").