[messaging] Minimal requirements for group chat

[Joseph Bonneau]

To try to summarize this thread so far, Michael proposed paring down
requirements slightly, specifically allowing O(N^2) communication for each
message and collision-resolution semantics, with the goal of keeping the
protocol simple. Kurt suggests that the requirement of having a consistent
total ordering on messages for everybody is the hardest goal left. Ximin
suggests partial ordering with flags to indicate partial ordering to users.
Moxie suggest that we can already do this with a simple protocol but the UI
(threaded messages) is then too complicated for a small screen (Tom later
provided some nice examples).

There appears to be a tension between UI from protocol complexity here. We
can imagine designing a simple protocol that requires a baroque UI with
messages threading and rejoining, or a very complicated protocol that
allows a simple UI (linearized messages).

Personally I'd lean towards the latter approach. This seems to be what
Michael's original thinking was, admitting a lot of messages and some
latency to allow for a simpler UI in the end. Though it's definitely worth
trying to design both cases, or consider if there's anything in the middle.
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