optimization: moved the exclusion of non-downloaded files from transmission logs to pre-cache stage. previously, exclusion took place after loading of the cache.
unit tests up-to-date with recent optimization
max passivity in playback (play.py, sequencer.py) reduced to 1 second. result of reducing passivity stored in cache, for efficiency. note: plots still reflect real timeline (non-reduced passivity), for aesthetic and conservative reasons.