Allow limiting the number of rayon threads

[cuviper]

Ref: rayon-rs/rayon#1227
cc @LelsersLasers

They are running Helix on a server with a high number of CPUs (208), but a relatively low number of processes (and threads) allowed per user -- ulimit -u is 512. The picker's rayon pool failed to start with ErrorKind::WouldBlock (EAGAIN).

Since nucleo explicitly sets num_threads for the pool, the general RAYON_NUM_THREADS environment variable doesn't work. I suggested using CPU affinity to limit std::thread::available_parallelism() instead, but that's not a great long-term solution to be forcing the use of particular CPUs for the entire process.

I see that Picker::new always passes None to get the default thread count, but I think it would make sense to have some configuration or similar environment variable so the user can override this.