Optimistic head closure

[ch1bo]

What & Why

Implement the protocol extension for more efficient Head closure as described in the Hydra Head paper.

This will make it faster & cheaper to close and finalize a Head as there is no need for a contestation phase.

What

• Close & finalize without contestation phase when every operator agrees (as also outlined in the original paper)

[ch1bo]

@Yasuke @chobbins This is very similar to the optimistic handling of closing channels in https://github.com/obsidiansystems/hydra-pay