(WIP) Persist sessions to ZooKeeper

[kmaehashi]

By applying this patch, JubaQL gateway now persists session records to ZooKeeper, so that:

• gateway instances can be clustered to provide high availability and performance
• session records can be recovered after restarting Gateway, even in non-HA configuration

In other words, this patch resolves SPoF issue of JubaQL system.

External changes introduced in this patch

• This patch adds the following options to Gateway.
  • --persist: enables session persisntence. When using this option, ZooKeeper hosts must be specified like -Djubaql.zookeeper=localhost:2181.
  • --gatewayId xxx: specify gateway cluster name. Runs in single node mode when omitted.
• This patch adds dependency to Apache Curator library, which is a client library for ZooKeeper.
• This patch changes the login protocol between Client and Gateway. When clients reconnect to the existing session, clients need to send login request (along with the session ID) to ensure that the session exists. (client side fix: Send session ID immediately when reconnecting to existing sessions jubaql-client#1)
• Session persistence creates ZooKeeper node under /jubaql/session.

TODOs

• This patch depends on use more distinguishable YARN application name #4. We'll rebase this patch later.
• Documentation must be updated.