Introduce post_scan method for workers

A worker can implement a post_scan method to receive stream info and
stream meta at the end of a stream.
As the end of a stream cannot be determined reliably, the post_scan
method can be called multiple times even before the stream actually
ends. It is also not guaranteed that the post_scan method is called when
the stream info or stream meta changes.
1 job for post_scan in 51 seconds (queued for 10 seconds)
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