Add test for (non-IO) message cancellation
Add test for (non-IO) message cancellation

The styx package can handle cancellation of non-Twrite/Tread requests,
because they do not block the decoding goroutine. We handle Twrite
requests in the connection's main goroutine to avoid copying a potentially
very large message, and we handle Tread requests for symmetry :). I
think we can get away with handling Tread in its own goroutine, but
Twrite requires some more creativity.

droyo/styx