Revision 1930

(SessionLoader.Destroy): New. Clear all
instance variables to cut cyclic references. The GC would have
collected the loader eventually but it can happen that it doesn't
run at all until thuban is closed (2.3 but not 2.2 tries a bit
harder and forces a collection when the interpreter terminates)
(load_session): Call the handler's Destroy method to make sure
that it gets garbage collected early. Otherwise it will be
collected very late if at all and it holds some references to e.g.
shapestores and the session which can lead to leaks (of e.g. the
temporary files)