BulletSim: fix corner case when rebuilding a compound linkset while
a mesh/hull while a mesh or hull is being rebuilt when its asset
is fetched. This fixes a 'pure virtual function' crash when changing
physical state of complex linksets that include many meshes.
BulletSim: default PhysicsTimeStep to same as the simulator's
heartbeat timestep when running the physics engine on a separate
thread. This reduces the occurance of heartbeats that happen when
there is no physics step which is seen as vehicle jerkyness.