tornado and tr/http_test.py: fix HTTPServer-related circular references.
Among other things, every web.RequestHandler object would end up leaking and
need to be picked up later by the garbage collector.
Add some checks to tr/http_test to make sure that these particular bugs
don't come back. Note that we *didn't* add a GcChecker to
http_test.HttpTest itself (the bulk of the tests) yet, because it still
leaks objects, albeit fewer than before. We can fix the rest in a future
4 files changed