Add stress test for Java worker

[raulchen]

What do these changes do?

Related issue number

[AmplabJenkins]

Test FAILed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/Ray-PRB/9641/
Test FAILed.

[robertnishihara]

@raulchen should we merge this as is or do you want to run it in the continuous integration?

EDIT: Ok, I guess it is being run. Travis is failing with

2018-11-28 06:19:17 INFO core [main]: Putting object 259effffaf87d63e50bc3edc3fd754dc07fcfed2, for task 531baea1af87d63e50bc3edc3fd754dc07fcfed2 
2018-11-28 06:19:17 INFO core [main]: Putting object 249effffaf87d63e50bc3edc3fd754dc07fcfed2, for task 531baea1af87d63e50bc3edc3fd754dc07fcfed2 
2018-11-28 06:19:17 INFO core [main]: Putting object 239effffaf87d63e50bc3edc3fd754dc07fcfed2, for task 531baea1af87d63e50bc3edc3fd754dc07fcfed2 
2018-11-28 06:19:17 INFO core [main]: Putting object 229effffaf87d63e50bc3edc3fd754dc07fcfed2, for task 531baea1af87d63e50bc3edc3fd754dc07fcfed2 
2018-11-28 06:19:17 INFO core [main]: Putting object 219effffaf87d63e50bc3edc3fd754dc07fcfed2, for task 531baea1af87d63e50bc3edc3fd754dc07fcfed2 
2018-11-28 06:19:17 INFO core [main]: Putting object 209effffaf87d63e50bc3edc3fd754dc07fcfed2, for task 531baea1af87d63e50bc3edc3fd754dc07fcfed2 
2018-11-28 06:19:17 INFO core [main]: Putting object 1f9effffaf87d63e50bc3edc3fd754dc07fcfed2, for task 531baea1af87d63e50bc3edc3fd754dc07fcfed2 
2018-11-28 06:19:17 INFO core [main]: Putting object 1e9effffaf87d63e50bc3edc3

The job exceeded the maximum log length, and has been terminated.

[raulchen]

@robertnishihara yep, all functions decorated by @Test will be run in CI.

[AmplabJenkins]

Test FAILed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/Ray-PRB/9642/
Test FAILed.

[raulchen]

Jenkins, retest this

[raulchen]

@robertnishihara somehow I couldn't re-run travis. but the failed jobs should be unrelated, because this PR doesn't touch any python code. It should be okay for merge now.