Add restore queue concurrency

Whenever you decide to restore something, the current queue system takes 1 item at a time, this can become problematic at times where as a user I want to restore a bunch of files and a database.

Let's say I start restoring 20 gigabyte of files, then shortly after requesting this task, I also need to restore a database (either related to the file restore or separate).

This will result in that the database restore will be queued, and first be processed when the large file-restore has completed.

As a hosting provider I would like to be able to define the amount of concurrent restore tasks that can run (either implementing it simple, saying 2 restore tasks, regardless of type), or take it to the next level and implement it so I can say 2 DB restores can run at same time, 3 full account restores, 1 email restore etc, and if you allow concurrency at all (like now).

This will give us (the hosting provider) the possibility to tweak these settings depending on our available resources, and possibly be able to process restores faster and not let the customer wait for one task to finish.