[NEW VERSION] Zookeeper 3.8.1

[sbernauer]

Which new version of Apache ZooKeeper should we support?

Please specify the version, version range or version numbers to support, please also add these to the issue title

Additional information

If possible, provide a link to release notes/changelog

Changes required

Are there any upstream changes that we need to support?
e.g. new features, changed features, deprecated features etc.

Implementation checklist

Please don't change anything in this list.
Not all of these steps are necessary for all versions.

• Update the Docker image Support zookeeper 3.8.1 docker-images#381
• Update documentation / examples / tests [Merged by Bors] - Support zookeeper 3.8.1 #689