message: Switch to blocking mutex lock
  Switch to a blocking mutex lock to prevent "Unable to ge transport" error in multi threaded applications.
  referenced issue
diff: @@ -32,7 +32,7 @@ impl<N> LoadBalancingStrategy<N> for RoundRobinSync<N> {
  /// Returns next node from a cluster
  fn next(&self) -> Option<&N> {
  - let mut prev_idx = self.prev_idx.try_lock();
  + let mut prev_idx = self.prev_idx.lock();
  if let Ok(ref mut mtx) = prev_idx {
  let next_idx = (**mtx + 1) % self.cluster.len();
  **mtx = next_idx;
  