tracked object data should be periodically flushed to disk

[astraw]

Currently, to save realtime 3D data to disk, we rely on either A) a call to stop saving data or B) a tracked object being killed. However if the mainbrain crashes hard, this can mean long trajectories that are currently being tracked get completely lost. Therefore, we should implement periodically flushing the currently tracked objects to disk without requiring them to be killed.