Robustified the logic for finding a track with a SyncSampleBox

Added IsoTrackBox.has_sync_sample_box(). This returns true of the track's
 MediaInformationBox contains a SampleTableBox that contains a
 SyncSampleBox.

Added IsoMovieBox.get_first_sync_track() and get_sync_tracks(). These use
 the IsoTrackBox.has_sync_sample_box() to find a sync track(s).

Reduced a lot of "message" logging to "debug" in IsoSampleTableBox.
 remove_sample_refs_before_point() and generally cleaned things up.