microRTPS: set trajectory_waypoint to be sent only, as this is outgoing data with vehicle_trajectory_waypoint_desired

 - IMU init requires valid published data (device ids, etc)
 - orb_group_count will include advertised instances before data is published, so this can't be used to throttle IMU init attempts

On NuttX with flow control, if no one was reading from the uart, the
close() call would block indefinitely waiting for data to be sent.

 - this is necessary if uORB::Subscription is used with static storage
duration