14726: Proposal for consensus node architecture update

[rbair23]

This PR defines a proposal for an updated consensus node architecture, with a special emphasis on the "consensus" half the of the node (the other half being "execution", where services live). This document does not describe the merkle tree, or reconnect, other other "platform" features that are actually part of "execution" (as per this document). Instead it focuses on Gossip, Event Intake, Event Creation, Hashgraph, and detecting and responding to dishonest nodes. These are the core modules that make up "consensus". Consensus can be defined as a black box that (mostly) takes transactions and produces rounds.