kiji-schema: SCHEMA-491. Share/reuse ZooKeeperClient instances when possible

issue: https://jira.kiji.org/browse/SCHEMA-491
review: https://review.kiji.org/r/1621
commit: https://github.com/kijiproject/kiji-schema/commit/f0906309d0723ddf840db12196aa2910eab50edb