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/TNonblockingServerTest.cpp b/lib/cpp/test/TNonblockingServerTest.cpp
index f2f5922..434217e 100644
--- a/lib/cpp/test/TNonblockingServerTest.cpp
+++ b/lib/cpp/test/TNonblockingServerTest.cpp
@@ -44,17 +44,17 @@
 using namespace apache::thrift;
 
 struct Handler : public test::ParentServiceIf {
-  void addString(const std::string& s) { strings_.push_back(s); }
-  void getStrings(std::vector<std::string>& _return) { _return = strings_; }
+  void addString(const std::string& s) override { strings_.push_back(s); }
+  void getStrings(std::vector<std::string>& _return) override { _return = strings_; }
   std::vector<std::string> strings_;
 
   // dummy overrides not used in this test
-  int32_t incrementGeneration() { return 0; }
-  int32_t getGeneration() { return 0; }
-  void getDataWait(std::string&, const int32_t) {}
-  void onewayWait() {}
-  void exceptionWait(const std::string&) {}
-  void unexpectedExceptionWait(const std::string&) {}
+  int32_t incrementGeneration() override { return 0; }
+  int32_t getGeneration() override { return 0; }
+  void getDataWait(std::string&, const int32_t) override {}
+  void onewayWait() override {}
+  void exceptionWait(const std::string&) override {}
+  void unexpectedExceptionWait(const std::string&) override {}
 };
 
 class Fixture {
@@ -63,7 +63,7 @@
     public:
       ListenEventHandler(Mutex* mutex) : listenMonitor_(mutex), ready_(false) {}
 
-      void preServe() /* override */ {
+      void preServe() override /* override */ {
         Guard g(listenMonitor_.mutex());
         ready_ = true;
         listenMonitor_.notify();
@@ -86,7 +86,7 @@
       listenHandler.reset(new ListenEventHandler(&mutex_));
     }
 
-    virtual void run() {
+    void run() override {
       // When binding to explicit port, allow retrying to workaround bind failures on ports in use
       int retryCount = port ? 10 : 0;
       startServer(retryCount);