ONOS-3411 Disconnect switches and stop stat collectors when openflow-base is being deactivated

Change-Id: I9a1b5fbb60a9e7135cd586fe717c18f83e3d864d
diff --git a/providers/openflow/group/src/main/java/org/onosproject/provider/of/group/impl/OpenFlowGroupProvider.java b/providers/openflow/group/src/main/java/org/onosproject/provider/of/group/impl/OpenFlowGroupProvider.java
index e69fd6b..6c646d0 100644
--- a/providers/openflow/group/src/main/java/org/onosproject/provider/of/group/impl/OpenFlowGroupProvider.java
+++ b/providers/openflow/group/src/main/java/org/onosproject/provider/of/group/impl/OpenFlowGroupProvider.java
@@ -128,7 +128,8 @@
     public void deactivate() {
         providerRegistry.unregister(this);
         providerService = null;
-
+        collectors.values().forEach(GroupStatsCollector::stop);
+        collectors.clear();
         log.info("Stopped");
     }