TMSS-1254: after discussion with the architect we opted for collapsing the scheduled_process_start/stop_time and scheduled_sky_start/stop_time into one more meaningful scheduled_start/stop_time. This timestamp specifies what we want. The actual_* timestamps the report what happened. And the plain timestamps report the combination of the two: what we want before running, and what happened after ending. Moved the timestamps also into the model as non-editable fields which are updated via database triggers. Added documentation.