Fixed Zookeeper to recursively create parent paths as necessary.

Review: https://reviews.apache.org/r/11366
Exposed version in "/vars" and "/state.json" endpoints.

Review: https://reviews.apache.org/r/11642
Fixed slave to not send tasks and executor info of a terminated executor,
when re-registering with master.

Review: https://reviews.apache.org/r/11543