Use ZooKeeperNet and log4net nuget packages

The ZooKeeperNet and log4net versions in the Kafka.Client project were out of date with what is nuget, so Kafka.Client wouldn't work in a project using those packages. This update just switched the solution to use the packages, with a couple of code changes to handle the update:
- KeeperException.GetCode() is now KeeperException.ErrorCode
- ZooKeeper.GetChildren(path, watch) returns IEnumerable<string> instead of List<string>