[SBN Read] Race condition in ObserverReadProxyProvider

There is a race condition in ObserverReadProxyProvider#getCurrentProxy():

  private NNProxyInfo<T> getCurrentProxy() {
    if (currentProxy == null) {
      changeProxy(null);
    }
    return currentProxy;
  }

currentProxy is a volatile. Another changeProxy() could occur after the changeProxy() and before the return, thus making the return value incorrect. I have seen this result in an NPE.