[CELEBORN-1421] Refine code in master to reduce unnecessary sync to get workers/lostworkers/shutdownWorkers.

[RexXiong]

What changes were proposed in this pull request?

1. Use ConcurrentSet to replace ArrayList for workers.
2. Remove unnecessary sync and snapshot when get workers/lostworkers/shutdownWorkers

Why are the changes needed?

1. Reduce unnecessary sync to get workers/lostworkers/shutdownWorkers.
2. Somewhere in the Master, directly using statusSystem.workers(ArrayList) is not safe, potentially leading to concurrent modification issues.

Does this PR introduce any user-facing change?

No

How was this patch tested?

Pass GA