ssl_stream does not use flat_stream

We no longer need to use flat_stream to improve write performance, as
Asio now linearizes scatter/gather I/O in ssl::stream:
https://github.com/chriskohlhoff/asio/commit/17637a48ccbfa2f63941d8393a7c8316a8df4a79

example/websocket/server/fast/websocket_server_fast.cpp

@@ -80,7 +80,7 @@ setup_stream(websocket::stream<NextLayer>& ws)
 //------------------------------------------------------------------------------
 
 void
-do_sync_session(websocket::stream<beast::tcp_stream> ws)
+do_sync_session(websocket::stream<beast::tcp_stream>& ws)
 {
     beast::error_code ec;
 
@@ -129,10 +129,10 @@ do_sync_listen(
         if(ec)
             return fail(ec, "accept");
 
-        std::thread{
-            do_sync_session,
-            websocket::stream<beast::tcp_stream>(std::move(socket))
-            }.detach();
+        std::thread(std::bind(
+            &do_sync_session,
+            websocket::stream<beast::tcp_stream>(
+                std::move(socket)))).detach();
     }
 }
 
@@ -441,11 +441,13 @@ int main(int argc, char* argv[])
     net::io_context ioc{threads};
 
     // Create sync port
-    std::thread{
-        do_sync_listen,
+    std::thread(beast::bind_front_handler(
+        &do_sync_listen,
         std::ref(ioc),
-        tcp::endpoint{address, static_cast<unsigned short>(port + 0u)}
-        }.detach();
+        tcp::endpoint{
+            address,
+            static_cast<unsigned short>(port + 0u)}
+                )).detach();
 
     // Create async port
     std::make_shared<async_listener>(