ENH: add MeshObject Release() static method

- Delete() will perform a 'checkOut()' which does the following:
  * remove the object from the registry
  * delete the pointer (if owned by the registry)

- Release() does the following:
  * transfer ownership of the pointer (if owned by the registry)

- Store() does the following:
  * transfer ownership of the pointer to the registry

ENH: use UPtrList of sorted objects for MeshObject updates

- few allocations and lower overhead than using a HashTable,
  ensures the same walk order over the objects (in parallel)

STYLE: adjust meshObject debug statements