#1519: segault on import solved.  Problem relates to #1520 (also closed in this commit).  Completion callback 
was being re-entered due to the thumbnail import code calling spin_event_loop() (which is a legacy aspect of the 
old-style import, which was serialized in the main event thread).  Additionally, the BatchImport object was 
being derefed and freed because the Workers/BackgroundJob system does not maintain a reference to the object 
with the callbacks; this has been remedied.