Investigate using EWMA latency score instead of last known latency score in BroadcastSession

[yondonfu]

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The first pass at stake/latency based O selection will use the last known latency score for each session. We should investigate whether using EWMA latency score would be better.

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Model EWMA latency score parameters and see if we would be better off using it instead of last known latency score. If so, we can calculate the EWMA latency score after receiving the transcode duration from SubmitSegment() and then attach the result to BroadcastSession.