Partially handle predicate attributes

This resolves the immediate need for `varint` predicates in queries.  It is not a complete/general solution for conversion of arbitrary attributes, which is going to be rather more invasive, since we'll not only need to track down everywhere they're converted to cql but also take care not to double-convert values.

I'm submitting this PR for consideration as a temporary solution, since it unblocks the current `tid` range query capability that we need in https://github.com/wikimedia/restbase.

Fixes #38