Initial review

TODO:
  - [x] Fix twisted plugin name
  - [x] Remove [stray debug statement](https://github.com/praekelt/unique-code-service/blob/develop/unique_code_service/api.py#L24)
  - [x]  Either separate pool creation into a different api method or change import api method to not respond with `201 CREATED` if the pool already exists
  - [ ] Investigate whether we are handling duplicate import and redeem requests safely (where the second request happens before the first has been processed)