Switch from erlcql to cqerl.

Unfortunately, erlcql only supports CQL3 over Cassandra binary protocol v1, while for recent Cassandras, we want v2. Thankfully, cqerl supports v2, and converting the cassandra_cql driver is reasonably simple.

This patch does just that - converts from erlcql to cqerl.

The change to `examples/cassandra_cql.config` can be avoided, if those deps are added to `escript_incl_apps` in `rebar.config`, but I think I'm starting to realize what that is good for, and why, so I opted to add it to the example instead. Let me know if I should move them elsewhere.