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.
