Reduce Beast dependencies by leveraging C++11 features:

* Remove beast::Atomic (RIPD-728):
  * Use std-provided alternatives
  * Eliminate atomic variables where possible

* Cleanup beast::Thread interface:
  * Use std::string instead of beast::String
  * Remove unused functions and parameters

* Remove unused code:
  * beast::ThreadLocalValue
  * beast::ServiceQueue
Remove legacy LoadTypes (RIPD-365)