(ws) Add a new ws sub-command, push_server. This command runs a server which sends many messages in a loop to a websocket client. We can receive above 200,000 messages per second (cf #235).

ws/CMakeLists.txt

@@ -50,6 +50,7 @@ add_executable(ws
   ws_http_client.cpp
   ws_ping_pong.cpp
   ws_broadcast_server.cpp
+  ws_push_server.cpp
   ws_echo_server.cpp
   ws_echo_client.cpp
   ws_chat.cpp