[OCIS] deleting multiple elements concurrently will not delete all the items.

[dpakach]

Try to delete multiple elements from the trash at the same time.

reproducing this manually is difficult.
The test to demonstrate this is in.
https://github.com/owncloud/core/blob/35c53c096f40dd15ae15940c589965379e921857/tests/acceptance/features/apiTrashbin/trashbinFilesFolders.feature#L112