Towards push notifications in OCIS

<!--HTML-->To make clients pick up changes to a shared file the etag of all recipients root folders needs to be updated. The current implementation in OCIS jails shares into a `/Shares` folder to calculate a dynamic etag, based on all accepted shares. This multiplies the stat requests made to the underlying storage system by the number of shares on every propfind. By letting the storage registry cache the root etag of every storage id we can reduce this number to one stat request per storage. This cache would not only allow the gateway to calculate the etag for any path based on the mount point of storages: it would allow sharing the stat cache for all clients, the ocis instance could dynamically adjust the cache timeout to react to system load, and storages providers can push etag changes to the storage registry to prepare for push notifications to clients.