At startup, make the WOPIService check if the discovery data exists in the KeyValue store.
If it doesn't exist, synchronously fetch the discovery XML file, load the data in memory and update the KeyValue store. In case of failure, this prevents the service from starting, thus possibly the whole server.
If it exists, load the data in memory.
We need a nuxeo.wopi.dicsovery.url nuxeo.conf property.
Default value: https://onenote.officeapps.live.com/hosting/discovery.
If the property is not set the WOPIService should not be available.
The periodic refresh/cluster invalidation part will be handled by