Refactor querying to work more generically while supporting pagination

[DRK3]

Make querying work with some sort of general, common querying concepts instead of just taking in the entire implementation-specific string.

A Bookmark() method was added to the iterator interface in order to make use of CouchDB query result pagination, but it's not really properly general enough for all storage types. Right now MySQL just returns a blank string since the concept doesn't apply there in the same way (it returns a boolean). Figure out how to refactor this to make this work in a nicer way.