upload/download status: add recently-finished operations

[warner]

The code currently keeps track of the results of the last 10 or 30 upload/download operations, for display on the "upload/download status" web page. The queue that it uses is appended when an operation starts, so it can display the most recently-started operations. It would be nice to be able to see the most recently-finished operations too: have another queue that is appended to when the operation completes. This would be especially useful for large files, which may have been started a long time ago (and therefore will have fallen off the recently-started queue).