To allow other checkpoint backends we extract a checkpoint interface
which future checkpoints can implement.
* Add checkpoint interface for custom checkpoints
* Create RedisCheckpoint to implement checkpoint interface
* Swap out hosie redis library for go-redis
Minor changes
* Allow configuration of Redis endpoint with env var `REDIS_URL`
* Replace gvt with govendor
The previous pipeline model required a lot of setup and abstracted away
the processing of records. By passing a HandlerFunc to the consumer we
keep the business logic of processing of records closer to the use of
the consumer.
* Add refactoring note and SHA to README
