[zookeeper] Upgrade ZK client to Curator

This PR upgrades the ZK client used by Chronos from the old and sometimes busted Twitter Commons Zookeeper libs to Apache Curator. This should address problems we've seen previously with master failover.

I tried to write tests to cover failure scenarios. If there are other scenarios I should test, please let me know!

/cc @brndnmtthws @florianleibert @mrdmnd @wmoss @jamesmay @guenter @solidsnack