Explicit configuration for rayon thread count

The rayon documentation says that the default number of threads is not defined in a future-proof way. Currently it is based on a RAYON_NUM_THREADS environment variable, if set, and otherwise uses the number of logical CPUs. Using the environment variable may or may not be stable in the future, but either way requires additional configuration outside ghostflow-director's main configuration file.

We should create a configuration file field to explicitly specify the thread count.