IF: Finalize network protocol for HotStuff

[arhag]

Determine which network messages we actually need and what is their structure.

Related: #1569.

With unification of HotStuff proposals with block headers, we decided that we do not need a new message for proposal. We can just use the existing signed_block network message. However, additional information related to the new consensus algorithm will still be necessary. Those will be captured in a block header extension, but the details of that new design have yet to be worked out which is why this issue still remains open.

We also decided that we do not need a new message for sending a high QC because other leaders should receive all the vote messages anyway to construct the QC themselves.

The only new message that needs to be added is one for the finalizer votes. The message structure for that is unlikely to change compared to what we have now, but we will finalize that structure with the redesign of the IF consensus algorithm.