Return 'invalidate' entries and close the cursor on relevant drop, dropDatabase, and renameCollection oplog entries

If someone was trying to use changeNotifications to keep a cache in sync with changes to a mongodb collection or database, certain operations would cause that cache to need to be invalidated, namely drop, dropDatabase, and renameCollection operations that affect the collection or database that notifications are being received on. When a change notification feed encounters one of those operations it should return an 'invalidate' entry and close the change notification cursor