Reduce memory leak on copy, cursor creation
Similar logic to 3199cc2a (!3501 (merged)) The current SPDocument structure includes finalization cycles, preventing GC from removing it even after it is no longer referenced.
This forcefully destroys the SPDocument, instead of fixing the structure (as we intend to stop using GC), and instead of replacing GC (so it can be targetted for 1.1.1)
See #2723 (closed).