@Override
    public boolean isServiceUser(final UserIdentity userIdentity,
                                 final UserIdentity serviceUserIdentity) {
        return delegate.isServiceUser(userIdentity, serviceUserIdentity);
    }
Delegates to the appropriate {@link ServiceUserFactory} based on what {@link ServiceUserFactory}
implementations are bound.
stroom-security/stroom-security-common-impl/src/main/java/stroom/security/common/impl/DelegatingServiceUserFactory.java