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/Host.java b/src/main/java/net/onrc/onos/core/topology/Host.java
index 05849c5..4e0c2e4 100644
--- a/src/main/java/net/onrc/onos/core/topology/Host.java
+++ b/src/main/java/net/onrc/onos/core/topology/Host.java
@@ -33,7 +33,7 @@
     public Iterable<Port> getAttachmentPoints();
 
     /**
-     * Gest the Host last seen time.
+     * Gets the Host last seen time.
      * <p/>
      *
      * @return the Host last seen time. (UTC in ms)