@Override
                    public void run(final Status status) {
                        onRpcReturned(Replicator.this.id, RequestType.Snapshot, status, header, getResponse(), seq,
                            stateVersion, monotonicSendTimeMs);
                    }
Returns the next in-flight sending index, return -1 when can't send more in-flight requests.

@return next in-flight sending index
jraft-core/src/main/java/com/alipay/sofa/jraft/core/Replicator.java