Make use of C++11 atomics

Now that we require C++11, we can make use of std::atomic. For example:

• libprocess/process.cpp uses a bare int + __sync_synchronize() for "running"
• __sync_synchronize() is used in logging.hpp in libprocess and fork.hpp in stout
• sched/sched.cpp uses a volatile int for "running" – this is wrong, "volatile" is not sufficient to ensure safe concurrent access
• "volatile" is used in a few other places – most are probably dubious but I haven't looked closely