Replace unstructured Task {} in MediaMixerOutput and StreamOutput
conformances with AsyncStream channels that preserve FIFO ordering.
The previous pattern created a new Task for each audio/video callback,
which provides no ordering guarantee when entering an actor's serial
executor. This caused adjacent frames to arrive out of order, resulting
in RTMPTimestamp.invalidSequence errors (silent frame drops) and
AVAssetWriter failures in StreamRecorder.
William Porter
shogo4405