Rework background job system

apps/user_ldap/appinfo/app.php

apps/user_ldap/lib/jobs.php

cron.php

lib/backgroundjob/job.php

lib/backgroundjob/joblist.php

lib/backgroundjob/queuedjob.php

lib/backgroundjob/queuedtask.php

lib/backgroundjob/regulartask.php

lib/backgroundjob/timedjob.php

lib/backgroundjob/worker.php

lib/base.php

lib/cache/fileglobalgc.php

lib/public/backgroundjob.php

tests/lib/backgroundjob/dummyjoblist.php

tests/lib/backgroundjob/queuedjob.php

tests/lib/backgroundjob/timedjob.php