Adds autodiscovery support to server-to-server sharing as specified in the specification. If no discovery data is found it is using the fallback ownCloud endpoints for legacy support.
The data from the share_external is not to much. Thus we enrich this
data with info from the filecache.
This allows endpoints using this to actually show usefull information.
The filecache might not be up to date but that is a sacrifice we need to
make in terms of speed. Else the number of remote PROPFINDS grows
lineary with the number of remote shares wich will make this endpoint
practically unusable.
* list incoming remote shares at 'remote_shares'
* get per share info at 'remote_shares/<ID>'
* delete remote share with a DELETE to 'remote_shares/<ID>'
