More fine grained filtering NACKs after a key frame. (#2159)

* More fine grained filtering NACKs after a key frame.

There are applications with periodic key frame.
So, a packet lost before a key frame will not be retransmitted.
But, decoder could wait (jitter buffer, play out time) and cause
a stutter.

Idea behind disabling NACKs after key frame was another knob to
throttle retransmission bit rate. But, with spaced out retransmissions
and max retransmissions per sequence number, there are throttles.
This would provide more throttling, but affects some applications.
So, disabling filtering NACKs after a key frame.

Introducing another flag to disallow layers. This would still be quite
useful, i. e. under congestion the stream allocator would move the
target lower. But, because of congestion, higher layer would have lost
a bunch of packets. Client would NACK those. Retransmitting those higher
layer packets would congest the channel more. The new flag (default
enabled) would disallow higher layers retransmission. This was happening
before this change also, just splitting out the flag for more control.

* split flag

pkg/sfu/buffer/rtpstats_receiver.go

@@ -315,7 +315,9 @@ func (r *RTPStatsReceiver) SetRtcpSenderReportData(srData *RTCPSenderReportData)
 		timeSinceFirst := srData.NTPTimestamp.Time().Sub(r.srFirst.NTPTimestamp.Time()).Seconds()
 		rtpDiffSinceFirst := srDataCopy.RTPTimestampExt - r.srFirst.RTPTimestampExt
 		calculatedClockRateFromFirst := float64(rtpDiffSinceFirst) / timeSinceFirst
-		if math.Abs(float64(r.params.ClockRate)-calculatedClockRateFromLast) > 0.2*float64(r.params.ClockRate) || math.Abs(float64(r.params.ClockRate)-calculatedClockRateFromFirst) > 0.2*float64(r.params.ClockRate) {
+
+		if (timeSinceLast > 0.2 && math.Abs(float64(r.params.ClockRate)-calculatedClockRateFromLast) > 0.2*float64(r.params.ClockRate)) ||
+			(timeSinceFirst > 0.2 && math.Abs(float64(r.params.ClockRate)-calculatedClockRateFromFirst) > 0.2*float64(r.params.ClockRate)) {
 			r.logger.Infow(
 				"clock rate skew",
 				"first", r.srFirst.ToString(),