fix to crash in client side VoxelSceneStats crash

This is NOT URGENT pre-demo.. a crash will only likely happen in odd cases where we get one stats packet from a server and then kill the node before getting a second stats packet.

There are a couple problems that contribute to this crash. This checkin fixes the following:

- Missing copy constructor for VoxelSceneStats causes potential crash on destructor
- Memory leaks of existing jurisdiction data on an unpackFromMessage() where if the stats already had a jurisdiction it would leak that data
- Memory leaks on multiple calls to sceneStarted()
- Fix Thread Safety issues between Application::nodeKilled() which is called on the main thread, and Application::parseVoxelStats() which is called on the voxel processing thread, each could modify the _voxelServerSceneStats map in size, and therefore they need locks