Update the DynamoDB checkpoint item to use the `dynamodbav` value for marshaling.
Fixes: https://github.com/harlow/kinesis-consumer/issues/142
Minor changes:
* Update the DDB example consumer to support a local version of DDB for streamlined testing
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`
