Fix batched queries with bounds on clustered key column

I don't think this fixes batching for all queries but it does improve the current situation a bit. By which i mean that it solves my current (blocking production) issue. :smile: A complete fix for correctly batching all queries will require, i think, implementing a CQL AST manager (a la, arel). Such an AST manager would be a Good Thing in it's own right as it would allow the construction of much more complex queries than are currently supported by Cequel.