Add string attributes to self-containd objects.

- Add common super class to *Event classes, which handles attributes.
- Add string attributes to *Event classes (ONOS-1564)
- Populate string attributes.
  Picked random attribute obtained from OF, just to initially populate attrs.
  Attributes to be used for each elements should be revisited later.
- *Impl class to use/reference self-contained objects
   prep-work for snapshot in mind.
- unified equals implementations
- Add unfrozen Copy constructor.
- Add freeze check to fixed attributes.
- Remove get*Impl which was not really adding value.

Change-Id: I10f9538f87d133a22237bd8ab97b8de421d3930b
diff --git a/src/main/java/net/onrc/onos/core/topology/StringAttributes.java b/src/main/java/net/onrc/onos/core/topology/StringAttributes.java
index 89c5b5c..16f69a8 100644
--- a/src/main/java/net/onrc/onos/core/topology/StringAttributes.java
+++ b/src/main/java/net/onrc/onos/core/topology/StringAttributes.java
@@ -2,6 +2,8 @@
 
 import java.util.Map;
 
+// TODO We may want to separate configuration attributes and
+//      running state attributes.
 /**
  * Interface for Elements with StringAttributes.
  */