Fixes to support Asio changes (API Change):

This adjusts Beast's interfaces and implementation to match
the changes in Boost.Asio.

example/websocket/client/async/websocket_client_async.cpp

@@ -40,8 +40,7 @@ fail(beast::error_code ec, char const* what)
 class session : public std::enable_shared_from_this<session>
 {
     tcp::resolver resolver_;
-    websocket::stream<
-        beast::tcp_stream<net::io_context::executor_type>> ws_;
+    websocket::stream<beast::tcp_stream> ws_;
     beast::multi_buffer buffer_;
     std::string host_;
     std::string text_;
@@ -50,8 +49,8 @@ public:
     // Resolver and socket require an io_context
     explicit
     session(net::io_context& ioc)
-        : resolver_(ioc)
-        , ws_(ioc)
+        : resolver_(beast::make_strand(ioc))
+        , ws_(beast::make_strand(ioc))
     {
     }