Fix video object tracking counter

[mzur]

When a video object tracking job is started, a cache value is incremented that is used to limit the number of concurrent tracking jobs for each user. The value is decremented, when the tracking job finishes or fails. The job class also has $deleteWhenMissingModels = true, though. This could delete a tracking job without decrementing the cache value (if the new annotation was deleted before the tracking job started). This could stack up so users won't be allowed to start new tracking jobs although none are actually running.

Remove $deleteWhenMissingModels from the job class and handle a missing annotation or use model manually. In all cases, the cache value must be decremented or deleted again.