* Avoid IO in event loop when removing backup
* Refactor backup size calculation
Currently size is lazy loaded when required via properties. This
however is blocking the async event loop.
Backup sizes don't change. Instead of lazy loading the size of a backup
simply determine it on loading/after creation.
* Fix tests for backup size change
* Avoid IO in event loop when loading backups
* Avoid IO in event loop when importing a backup
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