Fixes #6526

make sure that we don't try to access an already deleted files, fixes some file source not found warnings

Fixes #4593

Note that the root folder size is mandatory for quota calculation.

When two scanning processed run at the same time, for example when
scan.php and list.php were called at the same time on an external
storage mountpoint, duplicate insertion errors can occurs.

These errors are now logged and ignored.

Since both scans are running in parallel transactions, they don't see
each other's changes directly in the DB which can cause duplicate
insertion errors for mime types as well, but those mime types can't be
selected yet. The solution to this is to force-reload the mimetypes list
after the transaction is finished.

Fixes #5199

Conflicts:
	lib/private/vcategories.php