(TWILL-52) Improve ZK session expire handling. Avoid killing application if expiration happened by resorting to polling YARN for app status, until the AM is reconnected to ZK.

Signed-off-by: Terence Yim <terence@continuuity.com>
(TWILL-53) Expose DiscoveryServiceClient through TwillContext to allow TwillRunnable able to discover services announced within the same TwillApplication.

Signed-off-by: Terence Yim <terence@continuuity.com>