Shuffle the value numbers around in the database.

Shuffle the value numbers around in the database.

First, it's nice that for now we don't have any users yet, so we
can make incompatible changes to the database layout like this
without causing trouble. ;-)

There are a few reasons for this change. First, we now use value 0
uniformly as a timestamp for both mail and timestamp documents, (which
lets us cleanup an ugly and fragile bare 0 in the add_value and
get_value calls in the timestamp code).

Second, I want to drop the thread value entirely, so putting it at the
end of the list means we can drop it as compatible change in the
future. (I almost want to drop the message-ID value too, but it's nice
to be able to sort on it to get diff-able output from "notmuch dump".)

But the thread value we never use as a value, (we would never sort on
it, for example). And it's totally redundant with the thread terms we
store already. So expect it to disappear soon.