rate-limiting plugins don't use a batch statement anymore to increment
the counters, which used to overload a coordinator and generate a
warning. Since the new driver uses load balancing policies, we increment
the counters with regular queries.

Replace confusing `_execute()` by `execute()` for public usage, and
rename `_execute()` to `build_args_and_execute()`, for internal usage
only.

- The factory now retrieves TLS and authentication options for session
  and cluster spawning.
- The ngx stub is not require anymore inside of the CLI.
- Migrations don't use the `all` consistency anymore to allow Kong to
  start even if all C* nodes are not up.