Work toward ONOS-1451: Separate Event Key space per instance

Misc changes and cleanup (Step 2):
 * Added OnosInstanceId field to class TopologyEvent. For now it is
   just assigned, but is not used.

 * Minor refactoring of class TopologyEvent so it will be easier to make it
   immutable after its its internals and usage are inalized.

 * Modified the signature of method PathIntentMap.getIntentsByLink()
   to use LinkTuple for the lookup instead of LinkEvent.

 * Replaced (only inside onos/core/topology) the usage of Apache's
   Validate with Google's Preconditions for null-checking and assignment.

   With Apache's Validate we have to use two statements:
    Validate.notNull(foo);
    this.foo = foo;

   With Google's Preconditions we can do it with a single statement:
    this.foo = checkNotNull(foo);

   NOTE: Apache's commons-lang version 3.x adds Google's semantics.
   In the future, we need to decide (across all ONOS code) whether
   to use Google's Preconditions or Apache's Validate 3.x

 * Removed one of the LinkEvent convenience constructors, because it is used
   only in 1-2 places in the unit tests, and nowhere in the main code.

 * Few other (minor) cleanup changes

Change-Id: I05dae593ae1244a0af23515e7c73911f21275479
diff --git a/src/main/java/net/onrc/onos/core/topology/HostEvent.java b/src/main/java/net/onrc/onos/core/topology/HostEvent.java
index ebaf688..1c64939 100644
--- a/src/main/java/net/onrc/onos/core/topology/HostEvent.java
+++ b/src/main/java/net/onrc/onos/core/topology/HostEvent.java
@@ -11,6 +11,7 @@
 import net.onrc.onos.core.topology.web.serializers.HostEventSerializer;
 import net.onrc.onos.core.util.SwitchPort;
 
+import static com.google.common.base.Preconditions.checkNotNull;
 import org.codehaus.jackson.map.annotate.JsonSerialize;
 
 /**
@@ -42,11 +43,13 @@
         mac = null;
     }
 
+    /**
+     * Creates the Host object.
+     *
+     * @param mac the MAC address to identify the host
+     */
     public HostEvent(MACAddress mac) {
-        if (mac == null) {
-            throw new IllegalArgumentException("Host mac cannot be null");
-        }
-        this.mac = mac;
+        this.mac = checkNotNull(mac);
         this.attachmentPoints = new LinkedList<>();
     }