make sleep semaphore threshold configurable

I think we rashly merged !377 (merged). Mazstab using the xs problem size reports that using the semaphore greedy (EMPER threshold == 0) has significant (at least how interpret the plots no idea about the actual statistics) benefits for 5 out of 7 benchmarks.

matmul.pdf for example.

I don't trust those reasults because my rts-descriptor also uses sleep_sem_threshold=workerCount which should exactly be exactly the same as the default configuration.

emper:
 meson:
   options:
     sleep_sem_threshold: workerCount