added api for long-running background tasks
added api for long-running background tasks

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