Leader selection in federated hyperchains

Block (and microblock) assembly in a Hyperchain is a process that will involve *one* leader. This leader assembles the block and then proposes it to the other members of the mining coalition. This leader (or failing that, any of the other members) can then issue the block commit when enough members have signed. Selecting, and rotating when necessary, this leader is the heart of a traditional BFT algorithm. We can get away with an incremental approach to this problem: first, we can make leadership simply *configurable,* and then eventually replace with a BFT library.

Implementing leadership shouldn’t involve modifying much of the behavior of the `neon_node`: the leader is going to act the same as the miner does currently, except for the changes in the other multi-party mining issues (https://github.com/hirosystems/stacks-hyperchains/issues?q=is%3Aissue+label%3Afederation)