make AnnotationDB use weakValueDictionary caching with autoGC option
make AnnotationDB use weakValueDictionary caching with autoGC option

cjlee112/pygr,ctb/pygr,GenDataPro/pygr,ctb/pygr,ctb/pygr,GenDataPro/pygr,theoryno3/pygr,cjlee112/pygr,cjlee112/pygr,GenDataPro/pygr,ctb/pygr,theoryno3/pygr,RitwikGupta/pygr,ctb/pygr,cjlee112/pygr,RitwikGupta/pygr,theoryno3/pygr,RitwikGupta/pygr