Basic placeholder metavoxel save/load, added some stats on server and client, tie metavoxel LOD to avatars', spanner streaming fix.

This is the only place where MetavoxelServer::_persister is set (it is not initialized to NULL in the ctor).  Was this intentional (since MetavoxelServer is a ThreadedAssignment and can only be used in a certain way)? or did you forget to initialize it at the ctor?