Improve decentralization ratio

[lrettig]

As discussed in Discord, this number isn't currently very meaningful or useful and in fact is a bit misleading. Let's improve this or replace it with something else. One idea that's been suggested is using active coinbases, which is also imperfect but maybe better than the current metric. Another option is active, actual, unique node count -- we do have some data on this but it's subjective and not currently tracked by the backend. What else should we consider?