Shared worker group in WorkerGroupGetter should be closed

When benchmarking Ozone streaming pipeline using `ozone freon ockg`, the benchmark would not end although all the keys have been written. 

Using Arthas, found that non-daemon threads like "NettyClientStreamRpc-workerGroup–thread1" are still running even after the benchmark has finished. 

It seems that when hdds.ratis.raft.netty.dataStream.client.worker-group.share is set to true, it will create a worker group that will be shared. However, this worker group will never be closed causing the JVM shutdown hook to never be triggered. The benchmark was able to work normally if the share configuration is disabled.

Need to find a way to shutdown the worker group gracefully without losing the shared worker group benefit.