EZP-21825: Temporary files don't always get cleaned up, filling up disk space

Temporary files could be left on the local FS when the copy from DFS to FS is considered unsuccessful.

In `eZDFSFileHandlerMySQLiBackend::_fetch()` the filename of the temporary file changes in every iteration of the loop, but gets deleted only after the loop.

So, when the loop gets iterated 3 times and the copy process is still unsuccessful, 2 temporary files remain on the file system.

https://jira.ez.no/browse/EZP-21825

Cheers
:octocat: Jérôme
You're right, it makes more sense there. Since the `copyFromDFS` method is only used hera (I did a full text search), I could safely move the functionality to the backend. Which I will do right after posting this comment :).