Remove dependency on ZooKeeper

[ekoutanov]

By and large, the information we currently obtain from ZooKeeper can be had from the Kafka Admin API, sans a few attributes that are of little relevance (e.g. broker start time). There shouldn't be a compelling reason left to use ZK. Furthermore, some managed Kafka services disallow direct access to ZK for security reasons.

Once removed, also remove the ZooKeeper and Curator libraries.

Deprecate the zookeeper.connect property, printing a warning when it is used. (Eventually to be removed altogether.)