Provide in-memory data store implementation

The sole current kv store, LevelDbKeyValueStore, works well when the amount of data to be stored is prohibitively large to keep it all in memory. However, in cases where the state is small enough to comfortably fit in whatever memory is available, it would be better to provide an in-memory implementation. This can be backed by either a native Java class, or perhaps a Guava class, if that is found to scale better (or, of course, the backing implementation could be configurable).