lib: Strip comments and blank lines
session: Keep lock when creating session

This avoid a race condition if tryign to creat two sessions at the same time.
playlist: Keep lock when modifying session._emitters
playlist: Keep lock when modifying session._cache
docs: Update changelog
image: Keep lock when modifying image._callback_handles
playlist: Keep lock when modifying PlaylistUnseenTracks.{_num_tracks,_sp_tracks_len}
utils: Keep lock while modifying listeners