* Allow arbitrary validator set changes in DHB.
This replaces `NodeChange` with a full list of IDs and public keys,
instead of just a single to-be-added or to-be-removed node, to allow
completely replacing the set of validators by any arbitrary new set in a
single key generation step.
* Address review comments: added_nodes, comments.
* Fix MessageScheduler::First.
Make sure every node eventually gets to handle its messages.