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/EnumTest.cpp b/lib/cpp/test/EnumTest.cpp
index c935bc4..6487906 100644
--- a/lib/cpp/test/EnumTest.cpp
+++ b/lib/cpp/test/EnumTest.cpp
@@ -75,7 +75,7 @@
   BOOST_CHECK_EQUAL(EnumToString(MyEnumWithCustomOstream::CustoM2), "{2:CUSTOM!}");
 
   // some invalid or unknown value
-  MyEnum5::type uut = (MyEnum5::type)44;
+  auto uut = (MyEnum5::type)44;
   BOOST_CHECK_EQUAL(EnumToString(uut), "44");
 }