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/processor/ServerThread.h b/lib/cpp/test/processor/ServerThread.h
index 61e31a3..9cca2d6 100644
--- a/lib/cpp/test/processor/ServerThread.h
+++ b/lib/cpp/test/processor/ServerThread.h
@@ -35,7 +35,7 @@
  */
 class ServerState {
 public:
-  virtual ~ServerState() {}
+  virtual ~ServerState() = default;
 
   /**
    * Create a server to listen on the specified port.
@@ -105,9 +105,9 @@
   public:
     Helper(ServerThread* serverThread) : serverThread_(serverThread) {}
 
-    void run() { serverThread_->run(); }
+    void run() override { serverThread_->run(); }
 
-    void preServe() { serverThread_->preServe(); }
+    void preServe() override { serverThread_->preServe(); }
 
   private:
     ServerThread* serverThread_;