[core] Fix MaxBW limitation

[maxsharabayko]

Fixes #713.

1. Don't reschedule sending (keep pacing) on
   • SND drop,
   • NAK received
   • retransmission timeout.
2. Added pacing amendment after sending probing packets.

Changes extracted from #2180.

[maxsharabayko]

Test Environment

• 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

This PR

⚠️ 100 ms stats interval on the sender.

SRT master

⚠️ 100 ms stats interval on the sender.