[ONOS-5171] FlowRuleStore: add purgeFlowEntries

The FlowRuleStore exposes purgeFlowEntry(DeviceId),
which purges from the store by a specific device.

Add purgeFlowEntries, to purge entries from all devices.

Change-Id: I0ec50d2bbb8b39097189f9e332dc3efd281bc811
diff --git a/core/api/src/main/java/org/onosproject/net/flow/FlowRuleStore.java b/core/api/src/main/java/org/onosproject/net/flow/FlowRuleStore.java
index bb8104d..f1cb90c 100644
--- a/core/api/src/main/java/org/onosproject/net/flow/FlowRuleStore.java
+++ b/core/api/src/main/java/org/onosproject/net/flow/FlowRuleStore.java
@@ -111,7 +111,12 @@
      *
      * @param deviceId device id
      */
-    void purgeFlowRule(DeviceId deviceId);
+    default void purgeFlowRule(DeviceId deviceId) {}
+
+    /**
+     * Removes all flow entries from store.
+     */
+    void purgeFlowRules();
 
     /**
      * Updates the flow table statistics of the specified device using