SCHEMA-501. TableLayoutMonitor user tracker reports a multimap (user ID -> layout IDs).

- Update the way clients registers themselves as users of a table layout.
- Add TableLayoutMonitor.unregisterTableUser().
- Add and improve unit-tests for TableLayoutMonitor.
- Tiny fixes in ZooKeeperClient.

issue: https://jira.kiji.org/browse/SCHEMA-501
review: https://review.kiji.org/r/878