face+util: Add protected Face::getTransport method and simplify DummyClientFace implementation

Change-Id: I42e7e5705f27b6823ed18107b374c9841d2a7500
Refs: #3146
diff --git a/src/face.hpp b/src/face.hpp
index 9a01d17..9e6faa4 100644
--- a/src/face.hpp
+++ b/src/face.hpp
@@ -672,6 +672,13 @@
     return m_ioService;
   }
 
+NDN_CXX_PUBLIC_WITH_TESTS_ELSE_PROTECTED:
+  /**
+   * @brief Get the underlying transport of the face
+   */
+  shared_ptr<Transport>
+  getTransport();
+
 private:
 
   /**