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)
In other words, we encapsulate convertRow so that we never need to call it directly; we simply ask for the JS or the Cassandra value representations as needed.