New stdx namespace in server repository

Add a new source directory src/mongo/stdx and namespace "mongo::stdx::" in the server codebase to provide symbol indirection for boost/std symbols.

Motivation: When you switch from a C++03 compiler to C++11, a number of things you formerly wanted to use from boost:: are now provided by std::.

We should not sprinkle #ifdefs at call sites to deal with this. Instead, create a new namespace "mongo::stdx" (stdx = "std extended") and encapsulate #ifdefs there.

This was motivated by SERVER-13256 ("Remove pch.h")