Dual-homing probing improvements

* Extend timeout from 1500 to 3000 ms (including 1000 ms initial delay)
* Use unicast instread of multicast probe for IPv6 host

Change-Id: I6aa9d9b45502e06252596417c685fda6be28fdbb
diff --git a/core/store/dist/src/main/java/org/onosproject/store/host/impl/DistributedHostStore.java b/core/store/dist/src/main/java/org/onosproject/store/host/impl/DistributedHostStore.java
index 5f59921..436ec9b 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/host/impl/DistributedHostStore.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/host/impl/DistributedHostStore.java
@@ -108,7 +108,7 @@
     private Consumer<Status> statusChangeListener;
 
     // TODO make this configurable
-    private static final int PROBE_TIMEOUT_MS = 1500;
+    private static final int PROBE_TIMEOUT_MS = 3000;
 
     private Cache<MacAddress, PendingHostLocation> pendingHostsCache = CacheBuilder.newBuilder()
             .expireAfterWrite(PROBE_TIMEOUT_MS, TimeUnit.MILLISECONDS)