Move remaining calls to AVPlayerItem.currentDate() off main thread

[CFKevinRef]

AVPlayerItem.currentDate() can block the calling thread, presumably to wait for downloading and parsing of a playlist since it's based on EXT-X-PROGRAM-DATE-TIME. Initial rendition info gathering is particularly likely to hit this case.

• This is reusing some code that inferred the date at which an AVMetricEvent occurred. It's been moved to its own method inferProgramDate(at mediaTime: CMTime).
• The functionality to get event timing as of right now on an AVPlayerItem has been moved from an initializer of PlaybackEventTiming into an async extension method AVPlayerItem.currentTiming()

Resolves [NAT-268]

[snyk-io]

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