Problem: need actor handle for polling use cases

Solution: restore _actor method, which provides actor handle.
Problem: does not support PLAIN authentication

Solution: expand configuration parsing to support this. Note that
security setting must come before any bind settings, as these are
executed in order. So:

    mlm_server
        echo = installing PLAIN security mechanism
        security
            mechanism = plain

        echo = binding mlm_client test service to 'ipc://@/malamute'
        bind
            endpoint = ipc://@/malamute