Prevent adding a land object if it overlaps any existing objects that have not had their bitmaps adjusted.

This is to prevent an immediate problem in http://opensimulator.org/mantis/view.php?id=7035 where a development code bug occasionally overlays all the existing parcels with a blank parcel owned by the estate manager and to gather more data.
My guess is that this parcel is being created by the new code in LandManagementModule.GetLandObject(), probably some race between threads since this only happens occasionally.
Adds regression tests for this case and for parcel subdivide.