Use backside-api to proxy messages

Tests still don't pass (hard to mock some of the stuff), but this is a pretty decent first pass at integrating with the backside-api. On that note, it depends on backside/backside-api#1 for this to work.

This uses the backside-api interface to do both reads and writes, and we now support writes via this interface.

There is a bit of messiness in terms of translating between rabbitmq keys and there keys (the send we don't change, but probably should), but it all works.