Code & unit test for detecting hang Spark workers 

It tries to detect if any Spark workers are dead. Then this info is reported to the healthz service, which can be further used by other external service (like load balancer) to determine the health status of job server (and spark cluster)

Note: the change already contains the updates based on the previous code review (based on ooyala/spark-jobserver). 
(nit) can you format it like `alive: Int, dead: Int` instead?