DownloadCallbacks: Ensure end for every successful add_new_download

[jrohel]

For every successful (ended without exception) call of add_new_download callback, the end callback is called once.

The code relied on librepo to call the end callback just once for each lr_target. Which works under normal conditions. But if there is an interruption in the librepo download (error, signal received) librepo will not call end callbacks for targets that have not yet started downloading. However, the add_new_download callback has already been called for them. For example, if a higher layer allocated resources on add_new_download and deallocated on end, resources would be lost.

The new code tracks this and calls the appropriate end callbacks. Also, an assert has been added to monitor for any attempt to double call the end callback.

[kontura]

Since the updated unit tests pass even without the file/package_downloader changes can I ask when is the problem happening?

[jrohel]

Since the updated unit tests pass even without the file/package_downloader changes can I ask when is the problem happening?

I have added a description of the resolved problem to the commit and PR.
The added unit tests pass with the original code, because under normal conditions librepo will call the end callback. The problem is when the librepo download is interrupted (error, signal).

[kontura]

Great, thank you!