If we do - ZBigFileH objects just don't get garbage collected, and
sooner or later this way it leaks enough filedescriptors so that main
zope loop breaks:

    Traceback (most recent call last):
      File ".../bin/runzope", line 194, in <module>
        sys.exit(Zope2.Startup.run.run())
      File ".../eggs/Zope2-2.13.22-py2.7.egg/Zope2/Startup/run.py", line 26, in run
        starter.run()
      File ".../eggs/Zope2-2.13.22-py2.7.egg/Zope2/Startup/__init__.py", line 105, in run
        Lifetime.loop()
      File ".../eggs/Zope2-2.13.22-py2.7.egg/Lifetime/__init__.py", line 43, in loop
        lifetime_loop()
      File ".../eggs/Zope2-2.13.22-py2.7.egg/Lifetime/__init__.py", line 53, in lifetime_loop
        asyncore.poll(timeout, map)
      File ".../parts/python2.7/lib/python2.7/asyncore.py", line 145, in poll
        r, w, e = select.select(r, w, e, timeout)
    ValueError: filedescriptor out of range in select()

    $ lsof -p <runzope-pid> |grep ramh | wc -l
    950

So continuing 64d1f40b (bigfile/zodb: Monkey-patch for ZODB.Connection
to support callback on .open()) let's change the implementation to use
WeakSet for callbacks list.

Yes, because weakref to bound methods release immediately, we give up
flexibility to subscribe to arbitrary callbacks. If it become an issue,
something like WeakMethod from py3 or recipes from the net how to do it
are there.