Refactor all examples:

fix #575, fix #604, fix #608, fix #634, fix #712

All examples are rewritten:

* Using Best Practices
* Mostly self-contained
* New examples to complete the feature matrix
* The server-framework example is removed

include/boost/beast/websocket/impl/close.ipp

@@ -65,13 +65,8 @@ public:
     {
     }
 
-    void operator()()
-    {
-        (*this)({});
-    }
-
     void
-    operator()(error_code ec,
+    operator()(error_code ec = {},
         std::size_t bytes_transferred = 0);
 
     friend