add cancellation support while connecting, to speed up WebSocket::stop

ixwebsocket/IXSocketConnect.h

@@ -7,20 +7,25 @@
 #pragma once
 
 #include <string>
+#include <functional>
 
 struct addrinfo;
 
 namespace ix 
 {
+    using CancellationRequest = std::function<bool()>;
+
     class SocketConnect {
     public:
         static int connect(const std::string& hostname,
                            int port,
-                           std::string& errMsg);
+                           std::string& errMsg,
+                           CancellationRequest isCancellationRequested);
 
         static bool connectToAddress(const struct addrinfo *address, 
                                      int& sockfd,
-                                     std::string& errMsg);
+                                     std::string& errMsg,
+                                     CancellationRequest isCancellationRequested);
 
         static void configure(int sockfd);
     };