Optimize export stubs cache
Optimize export stubs cache

Because the in_ operator is slow with big number of items and the data
that we are pulling is relatively small, it is a good trade-off to use a
bit more memory for better export performance.

AleksNeStu/ggrc-core,AleksNeStu/ggrc-core,AleksNeStu/ggrc-core,AleksNeStu/ggrc-core