Rework state management

[AdvancedAntiSkid]

• Consider using CAS and RWMutex instead of force-locking on Future<T>.

• Consider improving Future state, e.g.: introducing union Failed(E) | Completed(T) | Pending

[AdvancedAntiSkid]

Partially implemented
Consider whether we should actually store state as a union like java does for CompletableFuture where the value is a single state encoded as either null (pending), Failure(Throwable) or Object(value)