This PR moves all exception classes from runtime module to api module. It's aiming to eliminate the confusion about ray exceptions. It means that Ray users don't need to touch runtime module when API programming after this PR.
Note that this should be merged onto 2.0.
This is the 1st PR to remove the code path of multiple core workers in one process. This PR is aiming to remove the flags and APIs related to `num_workers`.
After this PR checking in, we needn't to consider the multiple core workers any longer.
The further following PRs are related to the deeper logic refactor, like eliminating the gap between core worker and core worker process, removing the logic related to multiple workers from workerpool, gcs and etc.
**BREAK CHANGE**
This PR removes these APIs:
- Ray.wrapRunnable();
- Ray.wrapCallable();
- Ray.setAsyncContext();
- Ray.getAsyncContext();
And the following APIs are not allowed to invoke in a user-created thread in local mode:
- Ray.getRuntimeContext().getCurrentActorId();
- Ray.getRuntimeContext().getCurrentTaskId()
Note that this PR shouldn't be merged to 1.x.
Now the Java LongPollClient's is not singleton, and a new polling thread will be created within a new LongPollClient for per RayServeHandle. It will degrade the performance of Replica Actor. So we change the LongPollClient's polling thread to singleton.
In general, broadcasting changes to the replicas via the LongPollClient is hard to reason about (it circumvents our versioning semantics as there's no rolling update). Ideally we would only be using the LongPollClient to broadcast replica membership and nothing else.
