Fix concurrent ffmpeg segment racing

This is a nasty one. The failure mode is:

1. Request A started FFmpeg and waited for a segment.
2. Request B requested an earlier or far away segment.
3. Jellyfin thought FFmpeg should to restart at a different position.
4. Request B killed the existing transcoding job.
5. Killing that job cancelled the same token request A was using.
6. The cancellation produced http 500 to request A.

To fix this:

we lock transcoding job state changes and segment handling per playlist, and use a thread safe counter to track how many http responses are still using each job’s segments. A job is only stopped or replaced once that counter reaches zero.

MediaBrowser.MediaEncoding/Transcoding/TranscodeManager.cs

@@ -612,9 +612,9 @@ public sealed class TranscodeManager : ITranscodeManager, IDisposable
     /// <inheritdoc />
     public void OnTranscodeEndRequest(TranscodingJob job)
     {
-        job.ActiveRequestCount--;
-        _logger.LogDebug("OnTranscodeEndRequest job.ActiveRequestCount={ActiveRequestCount}", job.ActiveRequestCount);
-        if (job.ActiveRequestCount <= 0)
+        var activeRequestCount = job.DecrementActiveRequestCount();
+        _logger.LogDebug("OnTranscodeEndRequest job.ActiveRequestCount={ActiveRequestCount}", activeRequestCount);
+        if (activeRequestCount <= 0)
         {
             PingTimer(job, false);
         }
@@ -697,7 +697,7 @@ public sealed class TranscodeManager : ITranscodeManager, IDisposable
                 return null;
             }
 
-            job.ActiveRequestCount++;
+            job.IncrementActiveRequestCount();
             if (string.IsNullOrWhiteSpace(job.PlaySessionId) || job.Type == TranscodingJobType.Progressive)
             {
                 job.StopKillTimer();