Handle rollbacks II

[ch1bo]

What & Why

The hydra-node is directly connected to the Cardano network via the cardano-node. As we submit & observe blocks containing Hydra protocol transactions, these blocks can also be rolled back as Cardano is based on the Ouroboros consensus.

Within this feature we want to make the hydra-node robust against rollbacks having it re-establish the last-known Hydra Head state by re-submitting Hydra protocol transactions if needed. This might be unsuccessful, in which case clients of hydra-node need to be notified of the rollback via the API.

To improve user/developer experience we aim to also provide a "maturity" of the Hydra Head via the API to indicate a measure of "how settled" the Hydra Head is. When a Head is fully mature / settled, i.e. it is open for longer than x blocks on the Cardano network, the Head will 100% open until closed and clients can submit transactions to a Hydra Head without the need to handle rollbacks.

• [ ] Update rollback documentation