wait with a condition variable instead of a this_thread::sleep_for so that waiting can be cancelled when we stop/shutdown the data thread (see #151)

ixwebsocket/IXWebSocket.h

@@ -22,6 +22,7 @@
 #include <mutex>
 #include <string>
 #include <thread>
+#include <condition_variable>
 
 namespace ix
 {
@@ -140,6 +141,10 @@ namespace ix
         static const uint32_t kDefaultMaxWaitBetweenReconnectionRetries;
         uint32_t _maxWaitBetweenReconnectionRetries;
 
+        // Make the sleeping in the automatic reconnection cancellable
+        std::mutex _sleepMutex;
+        std::condition_variable _sleepCondition;
+
         std::atomic<int> _handshakeTimeoutSecs;
         static const int kDefaultHandShakeTimeoutSecs;