* init/job_class.h (JobClass): Added blocking_event member
* init/job_class.c (job_class_new): Initialize blocking_event to NULL
* init/event.c (event_finished): When unblocking a job, NULL its blocking_event
field
* init/job.c (job_emit_event): When not emitting due to threshold, attempt to
block job with its class's blocking_event