Re-implement futures to not use locks

[TobiasWrigstad]

One of the features of the PonyRT is that it is completely lock-free. We should not implement an unavoidable lock below user-level in the implementation of futures as we currently do. Lock-free solutions are more scalable that lock-full and Encore promises scalability.

I am not against supporting lock-full futures as an option in the deployment language. But the default future behaviour should not use locks.