Bug/30 server connection problem (#31)

* use threads instead of std::async, need to cleanup threads

* less buggy server connection per thread system

ixwebsocket/IXSocketServer.h

@@ -12,6 +12,7 @@
 #include <string>
 #include <set>
 #include <thread>
+#include <vector>
 #include <mutex>
 #include <functional>
 #include <memory>
@@ -63,6 +64,8 @@ namespace ix
         std::atomic<bool> _stop;
         std::thread _thread;
 
+        std::vector<std::thread> _connectionsThreads;
+
         std::condition_variable _conditionVariable;
         std::mutex _conditionVariableMutex;