Fix #2474. Create a test the gets simulation frames concurrently.
Fix #2474. Create a test the gets simulation frames concurrently.

radiasoft/sirepo,mkeilman/sirepo,radiasoft/sirepo,mkeilman/sirepo,radiasoft/sirepo,mkeilman/sirepo,mkeilman/sirepo,radiasoft/sirepo,radiasoft/sirepo