private void sendEmptyEntries(final boolean isHeartbeat) {
        sendEmptyEntries(isHeartbeat, null);
    }
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