Add gokv.Store implementation for ZooKeeper

[philippgille]

ZooKeeper is often mentioned alongside Consul and etcd.

These three work different than Redis, but can be used as distributed key-value store, especially for metadata.

• https://github.com/apache/zookeeper
• https://zookeeper.apache.org/
• https://cwiki.apache.org/confluence/display/ZOOKEEPER/ProjectDescription
• https://github.com/samuel/go-zookeeper
• https://godoc.org/github.com/samuel/go-zookeeper/zk