Dual-homing probing improvements

(1) Active probing mechanism in the following two scenarios
    (1-1) Probe all ports on the pair device within the same vlan (excluding the pair port) when the 1st location of a host is learnt
    (1-2) Probe again when a device/port goes down and comes up again
    * Introduce HostLocationProvingService
        - DISCOVER mode: discover potential new locations
        - VERIFY mode: verify old locations
    * Can be enabled/disabled via component config
    * Improve HostHandlerTest to test the probing behavior

(2) Fix an issue that redirection flow doesn't get installed after device re-connects

(3) Temporarily fix a race condition in HostHandler by adding a little bit delay

Change-Id: I33d3fe94a6ca491a88b8e06f65bef11447ead0bf
diff --git a/core/api/src/main/java/org/onosproject/net/host/HostLocationProbingService.java b/core/api/src/main/java/org/onosproject/net/host/HostLocationProbingService.java
new file mode 100644
index 0000000..56d056c
--- /dev/null
+++ b/core/api/src/main/java/org/onosproject/net/host/HostLocationProbingService.java
@@ -0,0 +1,48 @@
+/*
+ * Copyright 2017-present Open Networking Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.onosproject.net.host;
+
+import org.onosproject.net.ConnectPoint;
+import org.onosproject.net.Host;
+
+public interface HostLocationProbingService {
+    /**
+     * Mode of host location probing.
+     */
+    enum ProbeMode {
+        /**
+         * Append probed host location if reply is received before timeout. Otherwise, do nothing.
+         * Typically used to discover secondary locations.
+         */
+        DISCOVER,
+
+        /**
+         * Remove probed host location if reply is received after timeout. Otherwise, do nothing.
+         * Typically used to verify previous locations.
+         */
+        VERIFY
+    }
+
+    /**
+     * Probes given host on given location.
+     *
+     * @param host the host to be probed
+     * @param connectPoint the location of host to be probed
+     * @param probeMode probe mode
+     */
+    void probeHostLocation(Host host, ConnectPoint connectPoint, ProbeMode probeMode);
+
+}