Remove obsolete classes, disable unused code, and tidy up:
* Removed MultiSocket. Code that previously used the MultiSocket now uses
  a combination of boost::asio coroutines and CRTP.
* Sitefiles headers rolled up and directory flattened.
* Disabled Sitefiles use of deprecated HTTPClient.
* Validators headers tidied up.
* Disabled Validators use of deprecated HTTPClient.
Remove unused and obsolete classes and tidy up:
Many classes required to support type-erasure of handlers and boost::asio
types are now obsolete, so these classes and files are removed:
HTTPClientType, FixedInputBuffer, PeerRole, socket_wrapper,
client_session, basic_url, abstract_socket, buffer_sequence, memory_buffer,
enable_wait_for_async, shared_handler, wrap_handler, streambuf,
ContentBodyBuffer, SSLContext, completion-handler based handshake detectors.
These structural changes are made:
* Some missing includes added to headers
* asio module directory flattened
Reduce RippleD dependencies on Beast:
* Use static_assert where appropriate
* Use std::min and std::max where appropriate
* Simplify RippleD error reporting
* Remove use of beast::RandomAccessFile
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
[FOLD] Remove Range.h
[FOLD] Remove LinkedListPointer
[FOLD] Remove ScopedValueSetter
[FOLD] Remove Win32 Registry Stuff
[FOLD] Remove FileSearchPath
[FOLD] Trim SystemStats down
[FOLD] Remove beast::static_bassert
[FOLD] Remove BinaryEncoding.h
[FOLD] Remove BinaryEncoding
[FOLD] SystemStats
[FOLD] Remove UnsignedInteger/UnsignedIntegerCalc
[FOLD] Use std::string instead of beast::String
[FOLD] Remove gratuitous use of beast::Atomic
[FOLD] Remove beast::Atomic deps from beast::StaticObject
[FOLD] Remove beast::Atomic from LockFreeStack
[FOLD] LockFreeStack