Listing shares becomes slow

When shares accumulate listing shares becomes slow because the ocs implementation has to enrich every share with user and file metadata, causing one Stat and GetUser request per share. Currently, the stat cache is turned off. We should probably reenable the stat cache.

As a workaround setting `FRONTEND_OCS_STAT_CACHE_STORE=memory` or `redis` for distributed deployments will allow ocs to reuse stat requests for already statted resources.

In the code we could process the list of received and created shares concurrently.