Use getMongoDatabase
This is still needed because GridFS constructor requires a DB not a MongoDatabase.
Once GridFS is updated (Mongo 3.1) this can be eliminated.
Implementation of Notification Handler in MongoDB
We store it all in one table, with an epoch number held (like a idGen)
And entries based on epochs stored as first class records (so we can query by
epoch and record)
Called when we get a message to indicate that the cache needs updating
Since we don't know whether the notification is about a leaf or folder,
unregister both.