Add injection to convert from long to AttributeValue
Make base DynamoStore more generic

DynamoStore is now a Store[String, AttributeValue]. This allows
you to use a bijection combined with ConvertableStore to create
any more specific type of store that you desire.

The expected [String, String] store is available through the
class DynamoStringStore
Create a mergeable DynamoLongStore

DynamoLongStore is a MergeableStore[String, Long] which uses
the plus monoid. The plus is implemented in the backing DynamoDB
API as an update with the special ADD action which does atomic
addition using 0 as the starting value if the key hasn't been
seen before