cassandra_int_tests: Refactor CassandraConnection execute methods

[rukai]

CassandraConnection implementation changes:

• improves support for the scylla and cdrs-tokio driver backends, I think the only thing they are missing now is TLS support.
• greatly reduces duplicated logic across: batch, prepared and regular queries. 👀

CassandraConnection API changes:

• _fallible methods return more useful types:
  • On success they return the generic Vec of results instead of some cassandra_cpp specific type.
  • On failure they return the cdrs-tokio errors instead of cassandra_cpp errors because cdrs-tokio errors are nicer to construct and read from
• _expect_err methods are replaced with _fallible methods, this gives us a more composable API.
• The execute method is kept instead of just using execute_fallible because it provides a better error message on failure then just unwrap.
• The execute_batch method is kept instead of just using execute_batch_fallible because it provides better assertions.

[conorbros]

LGTM. Should probably rename those methods at some point, execute and execute prepared are a bit confusing.