Remove unused handler code

Going to start spiking out how the new read API might look, and want
to get completely away from the existing read API.
Merge pull request #19 from alphagov/feature/delete-read-api

Remove unused handler code
Minor internal refactoring and renamings

Noticed that I’ve used CPS for handleWriteRequest, so be explicit in the
name of things. Define an explicit type, since that seems like
self-documenting code.