Improve usability of futures

[drewdzzz]

Currently, futures have only two "states" - ready and not ready. If a future was extracted, it will become "not ready" forever. Introduction of "invalid" state (or two states, "invalid" and "exhausted") would make futures much more usable.

As for me, it would be perfect if we had a special move-only class Future - a user would be allowed to extract the response only once then.

See the comment for details.

Note that #27 suggests to re-design the futures - in the case this issue will become irrelevant.