More robust download
Queue: minimum wait is now configurable
Added option to skip purging mirror failures

If an error occurs during processing user / media, admin can now choose to keep
mirror for such downloads (if mirror's enabled). The reason behind this option
is to simplify debugging failed transmissions with MAL.
User agent contains MG version and is configurable
Added limit for max parallel requests
Added configurable URL-wise download retrying
Logger can work without files
Added URL logging to downloader

Affects cron-queue.php and process.php only
Added old logs removal (configurable)

Increased max log size to 12M