Trim down prefix list to things we are actually using.

I've decided not to try for sup compatibility at the leve of the
xapian datbase. There's just too much about sup's usage of the
database that I don't like, (beyond the embedded ruby data structures
there is redundant storage of message IDs, thread IDs, and dates (in
both terms and values)).

I'm going to fix that up in the database of notmuch, with some other
changes as well. (I plan to drop "reference" terms once linkage to a
thread ID through the reference is established.  I also plan to add
actual documents to represent threads.)

So with all that incompatibility, I might as well make my own prefix
values. And while doing that, I should try to be as compatible as
possible with the conventions described here:

http://xapian.org/docs/omega/termprefixes.html