Base class for all futures that get something. Get one element.
In the subclasses, you will get easy semantics.
onGetis called when a successful response has been received (not null)
onNotFoundis called, when a response has been received, but the answer is null, indicating that the searched key was not found
The subclasses differ by these orthogonal concepts:
UserIdfrom the DHT, but you want to return a