Create an in-memory implementation of the storage.Registers interface. As a preliminary step, split Registers into 2 interfaces: RegistersReader and the main interface (similar to TransactionResults)

Next, implement all methods.

The cache will be used concurrently, however, data will be written once and immutable. We can take advantage of this to avoid adding a lock by checking if data was written before reading/writing.

The value for FirstHeight should be passed in during construction, and the value for LatestHeight should be passed in during the call to Store.