Improve performance by sending command hashes in proposals

[johningve]

With higher payload sizes, we should be able to reduce the size of proposals and reduce the time consumed by creating partial signatures if we send a hash of each command instead of the whole command. This optimization only works for systems where clients broadcast commands to all replicas, such that all replicas can locate a command in memory given its hash. Thus, it might be best if we make this optimization optional in order to support systems where this is not the case. We would also need a way for a replica to fetch commands from the other replicas if it is missing some of the commands.

[jianyu-niu]

Hi Olsen,

We are working on a project in a similar direction. One important thing is that you should pay attention to malicious missing fetch. (Glad to see your new protocols) I also noticed that you have implemented Fast-HotStuff in your codebase:).