stream.segmented: add AwaitableMixin
stream.segmented: add AwaitableMixin

- Move SegmentedStreamWorker.wait to AwaitableMixin
- Make both SegmentedStream{Worker,Writer} inherit from AwaitableMixin

chhe/streamlink,bastimeyer/streamlink,bastimeyer/streamlink,streamlink/streamlink,streamlink/streamlink,chhe/streamlink