[communication] Message counter can panic in shutdown

[frankmcsherry]

The message counter that uses a Rust MPSC can panic in shutdown if its attempt to report messages fails (presumably because the other side has hung up). It isn't immediately obvious why the other end is shutting down if there are still messages to read, so perhaps there is more to diagnose here, but we've seen this in the wild in tests. Some more discipline about error cases and intended invariants would help here.