CLI argument for setting max number of viewer threads

[nikolausWest]

Since we started parallelizing the viewer, it can end up hogging too much CPU such that it interferes with other running processes.

Add a CLI argument that sets the max number of threads used by the viewer.

[emilk]

This will also be useful when profiling, because it is hard to get an overview of 16 threads in puffin.

We should perhaps default to max(2, num_cores - 2) rayon worker threads, saving one core for the main thread, and one core for the rest of the system.

So perhaps the default of -j is -2 where non-positive numbers means "num cored minus this".

[Wumpf]

RAYON_NUM_THREADS seems to be picked up since we're using the default rayon configuration

[nikolausWest]

I still think we should expose it as a CLI argument (and ideally an argument to rr.spawn as well) for the purpose of making it easy to find for our users. Users shouldn't need to know we use Rayon

[emilk]

This might be a good time for a re_parallel crate that handles the rayon initialization, so it is only initialized once.

Right now I think we don't initialize the thread pool explicitly when called from Python (meaning rayon threads are unnamed)