message: core: allow stopping msgs or default msg rates
  It makes sense to allow the message rate to be set to default or stopped
  entirely.
diff: @@ -1090,13 +1090,14 @@ SystemImpl::make_command_msg_rate(uint16_t message_id, double rate_hz, uint8_t c
  {
  MAVLinkCommands::CommandLong command{};
  - // If left at -1 it will stop the message stream.
  - float interval_us = -1.0f;
  + // 0 to request default rate, -1 to stop stream
  +
  + float interval_us = 0.0f;
  +
  if (rate_hz > 0) {
  interval_us = 1e6f / static_cast<float>(rate_hz);
  - } else {
  - LogErr() << "Rate(Hz) is invalid: %f" << rate_hz;
  - return std::make_pair<>(MAVLinkCommands::Result::UNKNOWN_ERROR, command);
  + } else if (rate_hz < 0) {
  + interval_us = -1.0f;
  }
  command.command = MAV_CMD_SET_MESSAGE_INTERVAL;
  