Remove beast::Atomic and friends

Only a few users of `beast::Atomic` where left, but the class (and its helper functions) contained a lot of platform-specific code. All that code is removed in favor of new C++11 constructs.

Please pay particular attention to the following to ensure correctness:
* 6ff9fd9: the use of `compare_exchange_strong`. 
* ba511fc: the replacement of `memoryBarrier` with `std::atomic_thread_fence`.

In both cases, I suspect that the memory ordering restrictions can be relaxed.

Reviews: @vinniefalco, @howardhinnant