ONOS-1981: Move expensive work off of event loop thread

Change-Id: I200d9b727a7d501d5b055574ddb0907b8f4756a2

@@ -613,11 +613,7 @@ public class DeviceManager
         }
     }
 
-    // Intercepts mastership events
-    private class InternalMastershipListener implements MastershipListener {
-
-        @Override
-        public void event(MastershipEvent event) {
+    private void handleMastershipEvent(MastershipEvent event) {
         if (event.type() != MastershipEvent.Type.MASTER_CHANGED) {
             // Don't care if backup list changed.
             return;
@@ -664,6 +660,20 @@ public class DeviceManager
             log.debug("Device is not yet/no longer in the store: {}", did);
         }
     }
+
+    // Intercepts mastership events
+    private class InternalMastershipListener implements MastershipListener {
+
+        @Override
+        public void event(MastershipEvent event) {
+            backgroundService.submit(() -> {
+                try {
+                    handleMastershipEvent(event);
+                } catch (Exception e) {
+                    log.warn("Failed to handle {}", event, e);
+                }
+            });
+        }
     }
 
     // Store delegate to re-post events emitted from the store.