Raw block storage for rollback buffer

[wolf31o2]

The UTxO RPC spec handles rollbacks differently than Ouroboros and will return "undo" blocks on a rollback, so if a rollback rolls back 3 blocks, it will replay those blocks in reverse order as an undo block until it rewinds to the correct point. This allows downstream consumers to be stateless, but it requires us to maintain some kind of persistent buffer to service these requests.