Improve performance of transcode&prepare tasks

Nobody has claimed this yet.

These tasks are unreasonably slow right now, probably because they transcode 1 segment at a time and our transcoding latency is not so much better than realtime (for a 4k video, I transcoded 10 minutes in 8 minutes).

We can improve this by parallelizing the transcoding process. This can be done easily by using go's concurrency abstractions. The only more complex part here is defining how much we're parallelizing without affecting the reliability of the transcoding infrastructure like the broadcaster, orchestrators, etc. The Bs might try to use the same O concurrently, which may not work well if we parallelize it in 5x or sth like that.

So the implementation here could be easy if it is just transcoding N segments at once or something like that, but picking the right value for N can be a complex task.

Start by locating the transcode and prepare task implementations and the current one-segment-at-a-time loop. Measure the existing throughput, then investigate Go concurrency limits and how parallel work could affect broadcasters, orchestrators, and Bs. Done means transcoding is faster without reducing infrastructure reliability, but the acceptable parallelism value is not specified.

Written by the indexing model from the issue text.