Remove lazy initialization of scheduler threads

[brson]

Each rust_scheduler has a fixed maximum threads, but only launches them lazily. This imposes a number of locking considerations and as I am intending to make major changes to the scheduler I would rather just remove this feature.

[brooksbp]

Would you simplify by launching max_num_threads in the rust_scheduler constructor instead of lazily in create_task?

EDIT: Reverting this #3498 ?

[brson]

@brooksbp yes, it means reverting #3498

[brson]

This isn't going to matter after all.