tornado and tr/ 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

Change-Id: Icd66b5fe1b775b80e6fc0f199cb2b89d60a57ee6
4 files changed