Better waitAll on scheduler

[dubzzz]

💡 Idea

While waitAll is not the best option as it tend not to be explicit on what the user want (mostly regarding the speed of scheduling), it's still the simplest option in many case.

We should make it able to wait less strictly. While playing around with it, it appears that the simplest option is probably to wrap calls with custom acts either

• not doing anything,
• doing an extra await,
• doing a sleep 0...

Well, these options might be directly offered on the scheduler at creation time.

[dubzzz]

Might be cool to have it before the next major, see #3891

[dubzzz]

Wait might not be the best name actually. Maybe we should have called it something like: run, release, unlock...