Add WebSocket 04 interface.

* New WebSocket04 traits class implements strategies.
* New "websocket_version" configuration setting selects between 0.2 and 0.4.

src/ripple/websocket/MakeServer.cpp

@@ -18,6 +18,7 @@
 //==============================================================================
 
 #include <BeastConfig.h>
+#include <ripple/basics/BasicConfig.h>
 #include <ripple/websocket/MakeServer.h>
 #include <ripple/websocket/WebSocket.h>
 
@@ -26,13 +27,16 @@ namespace websocket {
 
 std::unique_ptr<beast::Stoppable> makeServer (ServerDescription const& desc)
 {
-    static std::string const version = "0.2";
+    auto version = get<std::string> (
+        desc.config["server"], "websocket_version");
+    if (version.empty())
+        version = WebSocket02::versionName();
+
     WriteLog (lsWARNING, WebSocket) << "Websocket version " << version;
     if (version == WebSocket02::versionName())
         return makeServer02 (desc);
-
-    assert (false);
-    return {};
+    assert (version == "04");
+    return makeServer04 (desc);
 }
 
 } // websocket