Close an autopack race, where if a store was done after a sweep, but
before the collect_objs, the stored object would stay in packmark
until the next pack, preventing any objects it references from being
counted in the root-reachable test.

I don't have time to write a test case for this. :(

pack(): After we acquire the packlock, but before we set the packing
flag to True, we clear the packmark table.

_dopack(): Use the convenience wrapper _withlock method.