Remove all keys

Next phase of the Row update. Remove explicit (and some implicit) references to the Persistit Key class. The two main processes for this are: 

* Use `KeyCreator` interface more aggressively. Make the `StoreAdapter` not be a Key creator, but only the `Store` implementations. This makes many fewer places to correct when Key is replaced entirely. 
* Move more copy `Key` to/from `HKey` uses into the `ValuesHKey` implementation. There are still `Key`s thrown throughout the Adapter and Store layers, but that's for later work to remove.  
Stale comment or does it need a real creator here?