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)
Ah, I forgot to explain one tiny detail. The 'if' in generateSetConvertor() never resolves to true currently, as the object passed to the function is checked before the call. I put it there mostly as a safeguard in case somebody later on blindly calls it without performing sanity checks before.