Add pagination and return_terms support to Riak index queries

This builds on #845 and adds support for pagination and the `return_terms` parameter to the existing model and manager implementations.

The idea is to do this with minimal overhead. If it turns into a rabbit hole full of yaks, we'll need to find a different plan.
My thought here is just that we're going to be relying on this being a
particular kind of thing, so we should assert that. I'd also like to maybe
document it so that people don't have to go digging through lots of layers
of code to find out what to expect.