As we work towards introducing consumer groups to the repository we need a more generic name for the persistence layer for storing checkpoints and leases for given shards.
* Rename `checkpoint` to `store`
The Checkpoint functionality is an important part of the library and
previously it wasn't obvious that the Consumer was defaulting to Redis
for this functionality.
* Add Checkpoint as required param for new consumer
Major changes:
* Remove intermediate batching of kinesis records
* Call the callback func with each record
* Use functional options for config
https://dave.cheney.net/2014/10/17/functional-options-for-friendly-apis
Minor changes:
* update README messaging about Kinesis -> Firehose functionality
* remove unused buffer and emitter code
