FWaaS - Firewall Explicit Commit Operation

In Neutron Firewall as a Service (FWaaS), we currently support an implicit commit mode, wherein a change made to a firewall_rule is propagated immediately to all the firewalls that use this rule (via their firewall_policy association), and the rule gets applied in the backend firewalls. This might be acceptable, however this is different from the explicit commit semantics which most firewalls support. Having an explicit commit operation ensures that multiple rules can be applied atomically, as opposed to in the implicit case where each rule is applied atomically and thus opens up the possibility of security holes between two successive rule applications.