THRIFT-4762: Applied some C++11 refactorings to the runtime library and compiler (#1719)

* make use of C++11 override keyword
* added const specifier to TTransport::getOrigin()
* added more const correctness to the compiler
* make use of auto keyword
* replaced usage of NULL with nullptr
* make use of explicitly-defaulted function definition
* extended changelog
diff --git a/lib/cpp/test/TServerTransportTest.cpp b/lib/cpp/test/TServerTransportTest.cpp
index 539bd28..15177a8 100644
--- a/lib/cpp/test/TServerTransportTest.cpp
+++ b/lib/cpp/test/TServerTransportTest.cpp
@@ -34,11 +34,11 @@
 class TestTServerTransport : public TServerTransport {
 public:
   TestTServerTransport() : valid_(true) {}
-  void close() {}
+  void close() override {}
   bool valid_;
 
 protected:
-  shared_ptr<TTransport> acceptImpl() {
+  shared_ptr<TTransport> acceptImpl() override {
     return valid_ ? shared_ptr<TestTTransport>(new TestTTransport)
                   : shared_ptr<TestTTransport>();
   }