Review peerChurnGovernor policy for genesis

[coot]

Comment:

    -- TODO: #3396 revisit the policy for genesis
    increaseActivePeers :: ChurnMode -> STM m ()
    increaseActivePeers mode =  do
        modifyTVar peerSelectionVar (\targets -> targets {
          targetNumberOfActivePeers =
              case mode of
                   ChurnModeNormal  ->
                       targetNumberOfActivePeers base
                   ChurnModeBulkSync ->
                       min 2 (targetNumberOfActivePeers base)
        })