Modify Heterogeneous Paxos for Blockchains

[isheff]

This entails several sub-problems, including:

• Gossip Layer for Heterogeneous Paxos Transactions
• Message passing for Heterogeneous Paxos
• Modify Heterogeneous Paxos for Learner Graph Changes

Ultimately, we'll want an implementation of Heterogeneous Paxos that can run a blockchain state machine.