LocalStateQuery protocol: acquire state at implicit point

[coot]

It might be a good idea to acquire ledger state at an implicit Point (and report it back).

[dcoutts]

This would be an extra, alternative method of acquiring a state snapshot. We would also keep the ability to pick a specific point.

The use case for this method is when we do just want a recent tip, but the exact one does not matter. This is simpler since it avoids having to use the chain sync, and it avoids a race condition where the chain has moved on (e.g. switched fork) by the time we ask for a specific point.