[BEAM-5500] Fix memory leak in pickler.
[BEAM-5500] Fix memory leak in pickler.

We were storing reference to all dicts of modules for pickling,
assuming that the set of modules is bounded. It turns out that
not everything in sys.modules.values is a modue (or static).

rangadi/incubator-beam,RyanSkraba/beam,lukecwik/incubator-beam,apache/beam,lukecwik/incubator-beam,markflyhigh/incubator-beam,rangadi/beam,rangadi/incubator-beam,lukecwik/incubator-beam,apache/beam,robertwb/incubator-beam,mxm/incubator-beam,lukecwik/incubator-beam,markflyhigh/incubator-beam,robertwb/incubator-beam,apache/beam,markflyhigh/incubator-beam,RyanSkraba/beam,robertwb/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,rangadi/beam,lukecwik/incubator-beam,apache/beam,charlesccychen/beam,rangadi/beam,markflyhigh/incubator-beam,charlesccychen/beam,charlesccychen/beam,charlesccychen/beam,chamikaramj/beam,robertwb/incubator-beam,lukecwik/incubator-beam,RyanSkraba/beam,charlesccychen/beam,mxm/incubator-beam,robertwb/incubator-beam,iemejia/incubator-beam,RyanSkraba/beam,apache/beam,iemejia/incubator-beam,RyanSkraba/beam,apache/beam,RyanSkraba/beam,apache/beam,markflyhigh/incubator-beam,rangadi/beam,lukecwik/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,chamikaramj/beam,markflyhigh/incubator-beam,RyanSkraba/beam,charlesccychen/incubator-beam,rangadi/incubator-beam,charlesccychen/incubator-beam,rangadi/beam,rangadi/beam,chamikaramj/beam,apache/beam,charlesccychen/incubator-beam,apache/beam,lukecwik/incubator-beam,chamikaramj/beam,apache/beam,robertwb/incubator-beam,charlesccychen/beam,chamikaramj/beam,chamikaramj/beam,robertwb/incubator-beam,chamikaramj/beam,apache/beam,markflyhigh/incubator-beam,chamikaramj/beam,charlesccychen/beam,rangadi/beam,robertwb/incubator-beam,lukecwik/incubator-beam