API module (`ray/java/api` dir) includes all public APIs provided by Ray, it should be the only module that normal Ray users need to face.
The purpose of this PR to first improve the code quality of the API module. Subsequent PRs will improve other modules later. The changes of this PR include the following aspects:
1) Only keep interfaces in api module, to hide implementation details from users and fix circular dependencies among modules.
2) Document everything in the api module.
3) Improve naming.
4) Add more tests for API.
5) Also fix/improve related code in other modules.
6) Remove some unused code.
(Apologize for posting such a large PR. Java worker code has been lack of maintenance for a while. There're a lot of code quality issues that need to be fixed. We plan to use a couple of large PRs to address them. After that, future changes will come in small PRs.)
Currently, log directory in Java is a relative path . This PR changes it to `/tmp/raylogs` (with the same format as Python, e.g., `local_scheduler-2018-51-17_17-8-6-05164.err`). It also cleans up some relative code.
