Design/document the network layer used in the simulation

[daira]

The block download mechanism for Bitcoin's peer-to-peer protocol is summarized at https://developer.bitcoin.org/devguide/p2p_network.html .

We want to follow the general design of headers-first IBD and block relay, because we're trying to simulate performance (to within a constant factor) as well as functionality. We can make simplifications relative to Bitcoin, but they should be documented so that we can reason about how they affect any conclusions drawn from the simulation.