Reduce Beast public interface and eliminate unused code:

Beast includes a lot of code for encapsulating cross-platform differences
which are either not used by rippled or don't interact seamlessly with
new std version. Move away from custom implementations of features that
the standard library provides and reduce the number of platform-specific
features that Beast makes available.

Highlights include:
* Using std:: instead of beast:: when possible
* Reduce the use of beast::String in public interfaces
* Remove Windows-specific COM and Registry code
* Reduce the public interface of beast::File
* Reduce the public interface of beast::SystemStats
* Remove unused sysctl/getsysinfo functions
* Remove beast::Logger