Possible race condition trying to deserialize `Track` when performing `TranscodeJob`

[floehopper]

A while ago we were seeing ActiveJob::DeserializationError exceptions like this one. We tried to workaround it in this commit and then more successfully in this other commit, but we didn't tackle the root cause.

I suspect the problem is that we're using an after_save callback on Track to enqueue TranscodeJobs rather than using an after_commit callback as explained in this blog post.