Potential thundering herd HS upload once a day with hsv3

Here is a potential bug we found with dgoulet at HOPE.

In set_rotation_time() we set the rotation time of the next descriptor to the next midnight UTC with no randomization at all:

  service->state.next_rotation_time =
    sr_state_get_start_time_of_current_protocol_run() +
    sr_state_get_protocol_run_duration();

After descriptors get rotated, a new descriptor is built and uploaded, so it might be the case that all hsv3s upload that new descriptor at the same time at midnight UTC.

We should investigate more.