CXX-318 Don't rely on atexit shutdown by default

I don't love the name "initializer".  Especially since you can call Initializer::shutdown(). Maybe call it Driver?  It's basically the representation of the whole driver, that just happens to be a shell because you can only have one  instance per process and it's full of global data.  That name is crummy, too, though.  What do you think?