Updated networking requirements for Phase 0

[Mikerah]

Several months ago, @jannikluhn created this document about the networking requirements needed for Phase 0. It goes over the expected sizes of different data objects in Phase 0 and what latency and network throughput might look like.

Obviously, a lot of the spec has changed since then and thus we would need to update what some of these numbers might look like.

I am starting this issue to get a discussion going.

cc'ing @hwwhww, @mhchia and @ChihChengLiang