Implement superblock signing time bomb for Wit/2

[aesedepece]

The superblock signing committee from #2500 will be kept for the time being, on the condition that it will be eventually disabled and that signers will be sampled from the stakes tracker. This timed switch may be relative to the eventual activation date of 2_0.

[aesedepece]

We may need to discuss different strategies for sampling the committee out of the stakes tracker, which already provides functions for ranking, sampling and stuff.

[drcpu-github]

#2448