Fix error in timeout logic in Worker::wait_done() (#15)

This would cause excessive looping after first 10 seconds of execution.
On Windows it could halt execution entirely, depending on how threads get scheduled.

src/worker.cc

@@ -186,6 +186,10 @@ bool Worker::wait_all(int timeout_ms)
       {
         return false; // Return with timeout
       }
+      else
+      {
+        timeout = std::chrono::system_clock::now() + std::chrono::seconds(10);
+      }
     }
   }