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/concurrency/Tests.cpp b/lib/cpp/test/concurrency/Tests.cpp
index 019ae67..f2b0111 100644
--- a/lib/cpp/test/concurrency/Tests.cpp
+++ b/lib/cpp/test/concurrency/Tests.cpp
@@ -41,7 +41,7 @@
     args[ix - 1] = std::string(argv[ix]);
   }
 
-  if (getenv("VALGRIND") != 0) {
+  if (getenv("VALGRIND") != nullptr) {
 	  // lower the scale of every test
 	  WEIGHT = 1;
   }