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/EventLog.cpp b/lib/cpp/test/processor/EventLog.cpp
index e3ddbcc..c8e0d9b 100644
--- a/lib/cpp/test/processor/EventLog.cpp
+++ b/lib/cpp/test/processor/EventLog.cpp
@@ -110,7 +110,7 @@
 Event EventLog::waitForConnEvent(uint32_t connId, int64_t timeout) {
   Synchronized s(monitor_);
 
-  EventList::iterator it = events_.begin();
+  auto it = events_.begin();
   while (true) {
     try {
       // TODO: it would be nicer to honor timeout for the duration of this