introducing "slow queries"

In preparation for enabling queries over flexattrs, this is a new path that
lets queries avoid generating SQLite expressions altogether. Any query that
can be completely evaluated in SQLite will be, but when it can't, we now fall
back to running the entire query in Python by selecting everything from the
database and running the `match` predicate.

To begin with, this mechanism replaces RegisteredFieldQueries, which
previously used Python callbacks for evaluation. Now they just indicate that
they're slow queries and the query system falls back automatically.

This has the great upside that it lets use implement arbitrarily complex
queries without shoehorning everything into SQLite when that (a) is way too
complicated and (b) doesn't buy us much performance anyway. The obvious
drawback is that any code dealing with queries now has to handle two cases
(slow and fast).

In the future, we could optimize this further by combing fast and slow query
styles. For example, if you want to match with a substring *and* a regular
expression, we can do a first pass in SQLite and apply the regex predicate on
the results. Avoided for now because premature optimization, etc., etc.

Next step: implement flexattr matches as slow queries.
flexible attributes as slow queries

Queries over flexible attributes now Just Work!