api: Major API change.  OnInterest/OnData callbacks now use just references, not shared pointers

If shared pointer is necessary, it can be obtained using
.shared_from_this() on Interest or Data object.

This commit also corrects all internal uses of expressInterest/setIntersetFilter.

Change-Id: I20207a5789fd189902f2c6e3827260b6b27a2514
diff --git a/src/interest.hpp b/src/interest.hpp
index e09cf2c..c4d7bc6 100644
--- a/src/interest.hpp
+++ b/src/interest.hpp
@@ -18,7 +18,7 @@
 /**
  * An Interest holds a Name and other fields for an interest.
  */
-class Interest {
+class Interest : public enable_shared_from_this<Interest> {
 public:    
   /**
    * Create a new Interest for the given name and values.