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.

src/ripple/validators/impl/Tests.cpp

@@ -17,6 +17,7 @@
 */
 //==============================================================================
 
+#include <beast/module/core/maths/Random.h>
 #include <beast/unit_test/suite.h>
 
 namespace ripple {