Optimize RealPBFT setup

PR input-output-hk/ouroboros-network#1002 fixed the number of QuickCheck cases to 10, because each was taking about half a second. Based only on intuition as of yet, I think it's the construction of the genesis config that takes most of the time, since the prop itself is so simple.

This Issue is to investigate why it's slow. Two goals:

* undo/better justify input-output-hk/ouroboros-network#1002
* likely also speed up the rest of the RealPBFT properties