Clean up cache on server reinitialization
    
    https://fedorahosted.org/sssd/ticket/734
    
    We successfully detect when the server is reinitialized by testing
    the new lastUSN value. The maximum USN values are set to zero, but
    the current cache content remains.
    
    This patch removes records that were deleted from the server.
    It uses the following approach:
      1. remove entryUSN attribute from all entries
      2. run enumeration
      3. remove records that doesn't have entryUSN attribute updated
    
    We don't need to do this for sudo rules, they will be refreshed
    automatically during next smart/full refresh, or when an expired rule
    is deleted.