fix regressions with sleeping behavior of kinematic bodies

[sebcrozet]

• Kinematic rigid-bodies will no longer fall asleep if they have a nonzero velocity, even if that velocity is very
  small. The rationale is that, since that velocity is chosen by the user, they really want the platform to move even
  if the speed is low.
• Fix bug where kinematic bodies would ignore the wake_up flag passed to set_linear_velocity and
  set_angular_velocity.