RaftServerProtocolService may continue to reply after it is closed

This issue was observed while deploying Ozone with Ratis, with appendEntriesAsync AppendEntryRequestProto are sent to the followers in an asynchronous manner. It can happen that the follower is replying to the Leader even after the election is over.

         server.appendEntriesAsync(request).thenCombine(previous,
              (reply, v) -> {
              responseObserver.onNext(reply);
            current.complete(null);