Bugfix: Fix overlapping requests in files app

In some cases the files app tended to display the wrong resources when navigating quickly through the views. This happened because the resource provisioning step wasn't canceled.
This is now fixed by using vue-concurrency which on a high level wraps iterable generators which are cancelable. We're using it to wrap the resource loading and cancel it as soon as the resource set is not needed anymore.

It also improves the overall performance for the files app.

https://github.com/owncloud/web/pull/5917
https://github.com/owncloud/web/issues/5085
https://github.com/owncloud/web/issues/5875
