Revised SND DROP and SND pacing logic

[maxsharabayko]

Sending Priority

In the live streaming configuration with TL Packet Drop enabled original packet must have priority over packet retransmission because:

• The earlier the latest original packet reaches the receiver, the better the end-to-end latency constraint is kept as the receiver can detect too late packets earlier.
• If original packets are accumulating in the sender buffer, there is a network congestion, and likely most of the packet retransmissions won't reach the receiver. Therefore it is better to prioritize a more up-to-date payload, preserve the end-to-end latency and keep the jitter low. Once congestion is over, the streaming can be restored quicker.
• In-order but belated packets have a negative impact on jitter and end-to-end latency.

Proposed sending priority (for Live Configuration Only):

1. Retransmit a lost packet only if there is no original packet available for sending (which happens when the current sending rate < MaxBW).
2. Otherwise check if FEC packet can be sent.
3. Otherwise send the next original packet (first time sending).

Extracted Changes

Prefer Using Source Time if Provided Everywhere (Moved to #2185)

If a source time is provided, it must be considered as the origin time of a message. Meaning that buffer timespan calculation and the TL packet drop logic must rely on this time instead.
Thus there is no need to save two timestamps (m_tsOriginTime and m_llSourceTime_us). Just use m_tsOriginTime everywhere.

Refactored CUDT::packData() (Moved to #2229)

The logic which extracts an original (not ever sent yet_ packet from the sender buffer has been moved to a dedicated function CUDT::packUniqueData(..). The encryption order has been changed.

• Verify sending an encrypted does not have issues.

Fix SND Drop Logic (Moved to #2230)

Sender dropping logic is applied if the sender buffer timespan exceeds drop_threshold_ms.
Instead, the delay of the oldest packet in the buffer should be used (Tnow - OldestPacketTS).
This also correlates with the usage of the source time if provided to calculate the timestamp, which was not the case previously.

As before, only packets older than Tnow - drop_threshold_ms are dropped by the sender.

See issue #898, #1910.

Don't Suppress SND Pacing (Moved to #2232)

Don't allow to force sending if checkNeedDrop signals congestion.
Addresses #713.

[maxsharabayko]

Test Environment

• Two CentOS 7 test machines.
• Congestion: rate limit (LanForge).
• Sending rate (source bitrate): 7 Mbps.
• Network throughput:
  • 8 Mbps (+14%);
  • 5 Mbps (-28%) from 20th to 30th second.
• SRT Max BW limit: 8 Mbps (+14% from source bitrate).
• Packet loss: No (only by rate limit).
• RTT: 40 ms.
• SRT Latency: 150 ms.

(receiver)
./srt-xtransmit receive srt://:4200 --statsfile stats-rcv.csv --statsfreq 1s --metricsfile stats-metrics.csv \
                --metricsfreq 1s -v --enable-metrics

(sender)
maxbw=1000000
sendrate=7Mbps
./srt-xtransmit generate "srt://ip:4200?maxbw=$maxbw&latency=150" --sendrate $rate --duration 60s \
                --statsfile stats-snd.csv --statsfreq 100ms --enable-metrics -v

Vertsion	pktRecvTotal	pktRcvDropTotal	pktRecvBelatedTotal	pktRecvAppTotal	pktMissingAppTotal
this PR	37842	2229	196	36900	2229
master	38414	2397	935	36767	2397

Note. pktRecvAppTotal and pktMissingAppTotal are calculated by the receiving application (srt-xtransmit, stats-metrics.csv). Other statistics are calculated by SRT.

This PR

⚠️ 100 ms stats interval on the sender.

SRT master after PR #2232

⚠️ 100 ms stats interval on the sender.