Existing flows not cleared when Controller reconnects to the switch

When the controller reconnects to a switch with existing flows, it reads the flows into the oper state database. But since the flows are not in the config database, they can not be modified or deleted.

When reconnecting to a switch with existing flows, the controller must either "adopt" existing flows and import them into the config database, or impose its config database view on the switch (which is initially empty, since persistence is not supported).