Flows still reported in oper data store after they have been deleted from both config and network

Cleanup of flows that had been deleted from the network is undeterministic. With a larger number of flows that are removed fast (for example, if the entire config data space is deleted), there is always a random number of flows that are left in the oper space.

To reproduce, a mininet with 16-32 nodes can be used. Flows on the controller can be programmed and verified through the controller's REST API.

The procedure is as follows:

1. Add a large number of flows (e.g. 6000-10000) into the config space (REST).
2. Verify that the flows have been programmed into the network (use scripts that use the commands to dump flows from the ovs switches in the mininet network)
3. Verify that the Statistics Manager has detected the programmed flow, and that they are being reported in the oper space (there is currently another bug where the flows are not being properly discovered and they "appear" and "diappear - CONTROLLER-735)
4. Delete all flows in a single bolk operation (for example, by deleting the 'nodes' root in the config space)
5. Verify that the flows have been deleted from the network (use scripts that use the commands to dump flows from the ovs switches in the mininet network). The network will report no flows.
6. Check that a random number of flows is still being reported in the oper space.