Yield once to VM per quantum, another thread prio crate, some timings, use condvar instead of crossbeam; no improvements

[orottier]

Not really sure what to make out of all this. My CPU load goes down by a factor 2 but now my real-time issues are back.

Leaving this here for further consideration, my advices would be:

• keep padenot's thread prio crate, it looks solid
• keep the condvar, more lightweight than crossbeam and also some code improvements
• remove the tick processor and still yield to V8 continuously (old behaviour) - somehow my mac seems to go in deep sleep if the worker thread has nothing to do and may wake up only milliseconds later?