Add some sort of benchmarking utilities.

[vgarleanu]

It would be nice if we could use nightfall to benchmark a device and see how many/what kind of streams it could handle. We can achieve this by using the test files from https://jell.yfish.us/.

The benchmarking utility will download some test files, and spawn various transcoding sessions with various parameters. We can then record how many sessions we were able to spawn and what their transcoding speed is compared to the system load. We could also record data points like how long does it take to receive the first frames when seeking.