feat(machine): introduce backoff for handler timeouts

[pancsta]

• fixes fix(machine): add timeout confirmations #220
• add Backoff(), HandlerDeadline, HandlerBackoff

Fault tolerancy now works well under a heavy load with handlers timing out:

• handler is started
• timeout is reached (100ms)
• handle has to return within HandlerDeadline (10s)
• as early as Event.IsValid() returns true
• transition is canceled anyway
• in case of no return, the machine goes into a backoff state
• all the queue is flushed down the drain
• machine cancels any mutations for the period of HandlerBackoff (3s)
• new mutations run on a new event loop

This achieves goroutine cancellation based on probabilities (similar to hash collisions), and can be adjusted on per-machine and per-usecase basis.