How to update time_since_can_sleep in RigidBodyActivation?

[foxzool]

default_time_until_sleep() 2.0 is too long , I need to set it to more shorter.

[ThierryBerger]

You can modify time_until_sleep through activation_mut, but it's per entity.

I'm keeping this open because it's not trivial to discover, and we could provide a way to change the default.