New score type: distance from known winning route

[Zannick]

This would be a number of steps from any state on that route. We'd have to:

• Store this number in the statedb.
• Update states seen as they come in.
• Create a new ScoreMetric.
• Update recreated states as part of recording a solution.

It may be infeasible to exclude states from dropped solutions.

Also, this might update states a lot faster than we've previously done, so it may be wise to add more columns instead of shoving the entire StateData type into a single column.