Gather topology element info to TopologyImpl

- Moved all the self-contained topology elements (*Event) to
  TopologyImpl. (ONOS-1651)
  Now {Switch, Port, Link, Host}Impl is just a handler attached to
  TopologyImpl.
- BugFix: TopologyManager.addHost(HostEvent)
  HostEvent could be pushed to reorder queue multiple times,
  if multiple attachment point was given.
- BugFix: TopologyManager.{addLink, removePort}
  Properly handle if Host attachment point was removed as side-effect.
- BugFix: Copy HostEvent#lastSeenTime
- BugFix: Event instance notified to listeners (api*Events) should be
  the event which was/will be in the replica. (TopologyManager)
- Added/Modified debug log in TopologyManager so that log will be in
  same format for each event type.
  "{Added, Update, Removed} <Self-contained>"
- Removed backdoor method and use TestUtils instead.

Change-Id: If053d6f11f39574a188e7a52cb6194114f8afe5d
diff --git a/src/main/java/net/onrc/onos/core/topology/Port.java b/src/main/java/net/onrc/onos/core/topology/Port.java
index 4758b16..50feb99 100644
--- a/src/main/java/net/onrc/onos/core/topology/Port.java
+++ b/src/main/java/net/onrc/onos/core/topology/Port.java
@@ -120,11 +120,10 @@
      */
     public Collection<Link> getIncomingLinks();
 
-    // XXX Iterable or Collection?
     /**
      * Gets all the devices attached to this port.
      *
      * @return {@link Host}s attached to this port
      */
-    public Iterable<Host> getHosts();
+    public Collection<Host> getHosts();
 }