Track the most recently active validators and sample the superblock committee from them

[drcpu-github]

This PR will allow to select a number of superblock voters from the set of recently active validators. Note that this may require a bootstrap committee when we switch to PoS since there will possibly be very few active validators at that time.

[aesedepece]

I'll give this a thought on how to best integrate on top of wit/2

[drcpu-github]

I'll give this a thought on how to best integrate on top of wit/2

IMHO, this is one of the many small things that is required to strengthen wit/2 against griefing attacks. I'm by no means saying the implementation is the best approach, but the concept is definitely needed.