KIJIMR-52. Uses EntityId for keys in KijiTableKeyValueStore

Addresses [KIJIMR-52](https://jira.kiji.org/browse/KIJIMR-52) as I interpretted it.

Given no existing integration tests for the use of the KijiTableKeyValueStore, the changes to support EntityId were quite straightforward.

I'm happy to sign a contributors agreement as well.  Just send it along.