SCHEMA-387. Provide validators for schema evolution

issue: https://jira.kiji.org/browse/SCHEMA-387
review: https://review.kiji.org/r/578
SCHEMA-384. Track clients of a Kiji instance and clients of opened Kiji tables

 - ZooKeeperClient that handles ZooKeeper disconnects.
 - Adds a KijiTableMonitor : early design.

issue: https://jira.kiji.org/browse/SCHEMA-384