mosh-server can generate high traffic if a client can't hear it

Tonight's discovery:
If:

• a server is has an active application running (a tmux session that is being used from another client, for example) and the session generates significant changes
• the server can hear the client but the client does not hear the server

then the server's message traffic to the client can grow somewhat high-- a bandwidth graph I was looking at showed spikes to 1Mb/s of traffic for each update, whatever that means.

Given mosh's design, this isn't unexpected, but seems a little suboptimal for pay-by-use networks. I saw this while doing a router/server upgrade and fixed the routing problem, but this might be worth a little time to review/improve.

The report names no files, tests, or entry points. Start by reproducing the asymmetric connection with an active tmux session and substantial output, then trace how mosh-server handles outbound updates when the client cannot hear them. Done would require an agreed change that prevents unnecessary traffic without breaking the session.