ref #1616 [Increasing latency depending on hub remoteness]

http://redmine.named-data.net/issues/1616
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- refined and fixed playback timing adjustment code
ref #1616 [Increasing latency depending on hub remoteness]

http://redmine.named-data.net/projects/ndnrtc/issues
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- adjusted buffering phase
- added more precise playback latency tracking for ntp-synced
producer/consumer
'issues/1616-growing-latency' -> 'issues/1597-av-sync'
ref #1597 [Implement A/V synchronization v2]

http://redmine.named-data.net/issues/1597
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- now only audio is synchronized around the video in those cases:
 - if audio lags video for more than 45ms
 - if audio leads video for more than 15ms
'issues/1597-av-sync' -> 'issues/1616-growing-latency'