Proper conversion to/from JS when issuing queries to Cassandra

Until now, values were only sporadically converted between rb-cassandra and Cassandra types. This PR resolves this, and systematically converts values from JS to Cassandra when issuing queries, and back to JS after fetching the result rows. It also introduces conversion schemes for set values whose underlying types need conversion (such as set<varint>, set<josn>, etc.).

This PR aims at resolving [T84900](https://phabricator.wikimedia.org/T84900)
Okay, I guess it's a personal preference thing. @earldouglas: Please break the tie ;)