BulletSim: add locking of PhysObjects while processing simulation

step updates and collisions. This is an attempt to fix a crash reported
by Justin when doing high velocity teleports.

OpenSim/Region/Physics/BulletSPlugin/BSScene.cs

@@ -638,6 +638,8 @@ public sealed class BSScene : PhysicsScene, IPhysicsParameters
         lock (CollisionLock)
         {
             if (collidersCount > 0)
+            {
+                lock (PhysObjects)
                 {
                     for (int ii = 0; ii < collidersCount; ii++)
                     {
@@ -651,12 +653,15 @@ public sealed class BSScene : PhysicsScene, IPhysicsParameters
                     }
                 }
             }
+        }
 
         // If any of the objects had updated properties, tell the managed objects about the update
         //     and remember that there was a change so it will be passed to the simulator.
         lock (UpdateLock)
         {
             if (updatedEntityCount > 0)
+            {
+                lock (PhysObjects)
                 {
                     for (int ii = 0; ii < updatedEntityCount; ii++)
                     {
@@ -670,6 +675,7 @@ public sealed class BSScene : PhysicsScene, IPhysicsParameters
                     }
                 }
             }
+        }
 
         // Some actors want to know when the simulation step is complete.
         TriggerPostStepEvent(timeStep);