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/LinkEvent.java b/src/main/java/net/onrc/onos/core/topology/LinkEvent.java
index 00b4b90..f7ae6d3 100644
--- a/src/main/java/net/onrc/onos/core/topology/LinkEvent.java
+++ b/src/main/java/net/onrc/onos/core/topology/LinkEvent.java
@@ -9,7 +9,7 @@
 import net.onrc.onos.core.util.PortNumber;
 import net.onrc.onos.core.util.SwitchPort;
 
-import org.apache.commons.lang.Validate;
+import static com.google.common.base.Preconditions.checkNotNull;
 import org.codehaus.jackson.map.annotate.JsonSerialize;
 
 /**
@@ -42,9 +42,7 @@
      * @param id link tuple to identify this link
      */
     public LinkEvent(LinkTuple id) {
-        Validate.notNull(id);
-
-        this.id = id;
+        this.id = checkNotNull(id);
     }
 
     /**
@@ -74,19 +72,6 @@
     }
 
     /**
-     * Creates the Link object.
-     *
-     * @param srcDpid source switch DPID
-     * @param srcPortNo source port number
-     * @param dstDpid destination switch DPID
-     * @param dstPortNo destination port number
-     */
-    public LinkEvent(Dpid srcDpid, PortNumber srcPortNo,
-                     Dpid dstDpid, PortNumber dstPortNo) {
-        this(new LinkTuple(srcDpid, srcPortNo, dstDpid, dstPortNo));
-    }
-
-    /**
      * Gets a {@link LinkTuple} that identifies this link.
      *
      * @return a LinkTuple representing the Port