Upgrade C++ code to C++14 standard

Now the various builder compilers have been upgraded to versions supporting C++14 (CBD-2151), we should switch over to C++14 in our C++ codebase.

At a high level we need to:

1. Update required language version (https://crascit.com/2015/03/28/enabling-cxx11-in-cmake/).
2. Update all code to compile and pass unit tests.
3. Remove any code which is now longer needed (e.g. backports from C++14).