Background create strip namespace twice from path

CreateBuilderImpl::sendBackgroundResponse strips namespace twice:

• One in body of sendBackgroundResponse or newly introduced createResponseEvent.
• One in construction of CuratorEventImpl.

This could cause incorrect resulting path if we are creating "/zoo" under "/zoo" namespace.

This recall me ZOOKEEPER-4601, I verified that CuratorFramework::getConfig get "keeper/config" in "/zoo" and "/config" in "/zookeeper" namespace. We might need extra coordination with ZooKeeper community to maintain consistent between curator and zookeeper but I think we could fix `CreateBuilderImpl` independently. I remembered that I sent email in zookeeper dev list before, I will bump that up for discussion.