We have made many improvements to connection handling in Zuul.
Bring those back to Nodepool by copying over the zuul/zk directory
which has our base ZK connection classes.
This will enable us to bring other Zuul classes over, such as the
The existing connection-related code is removed and the remaining
model-style code is moved to nodepool.zk.zookeeper. Almost every
file imported the model as nodepool.zk, so import adjustments are
made to compensate while keeping the code more or less as-is.