If several cdn file parts hashes are received in getCdnFileHashesDone
and some middle one of them cancels the entire loader (for example
because of a file write error) a !_finished assert violation happens.
Regression was introduced in 2fa2fa41c5.
In file download failed handler we suggest to try to load the file
once again to the same location. After some changes we started to
forget filename before failed handler. That resulted in large files
loading to memory instead of hard drive.
Add a precondition in FileLoader to prevent such bugs in the future.
Regression was introduced in 835b1801bc.
We need to destroy all FileLoader instances before destroying the
Downloader instance, because they hold pointers to it and call its
methods in destructor if they need to cancel some MTP requests.
John Preston