sbus: support multiple interfaces on single path
    
    This patch removes the old message handler which is replaced
    with a new one that supports multiple interfaces registered
    on single object path.
    
    A hash table is used to store registered object paths and their
    interfaces. When an entry or the table itself is destroyed,
    registered object path is unregistered through delete callback.
    
    It temporarily removes support of Introspect and Properties
    standard D-Bus interfaces and disables unit tests of those
    interfaces. The support is brought back by following patches.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2339
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>