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/data.hpp b/src/data.hpp
index dc6d9f0..d6aba80 100644
--- a/src/data.hpp
+++ b/src/data.hpp
@@ -18,7 +18,8 @@
 
 namespace ndn {
   
-class Data {
+class Data : public enable_shared_from_this<Data>
+{
 public:
   struct Error : public std::runtime_error { Error(const std::string &what) : std::runtime_error(what) {} };