Closes coder/internal#253.

The Tunnel closing mechanism encounters a race if the passed io.ReadWriteCloser is shared between the Tunnel and the Manager, specifically if the closing of the Conn by one causes a read fail on the other.

The sequence of events during the flakey tests is as follows:

1. Tunnel sends stop response
2. Tunnel closes sendch
3. net.Pipe gets closed by Tunnel's sendLoop ending
4. Manager's serdes reads the response successfully and passes it to the speaker via the recvCh.
5. The speaker tries to deliver the response via the message's respCh. The respCh is buffered so this is non-blocking.
6. The recvLoopDone channel is closed by the recvCh getting closed (by failing to read from the closed conn)
7. unaryRPC is finally scheduled to run, where both of these conditions in a select block are ready, and so whether or not the response is returned is random.

[...]
	case <-s.recvLoopDone:
		logger.Debug(s.ctx, "recvLoopDone while waiting for response")
		return resp, io.ErrUnexpectedEOF
	case resp = <-respCh:
		logger.Debug(s.ctx, "got response", slog.F("resp", resp))
		return resp, nil

The solution is to therefore make the respCh unbuffered. That way, it's not possible for the recvLoopDone channel to be closed until the respCh is read from.