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