private void truncate() {
            if (buf.capacity() > DISCARD_LIMIT) {
                valueUpdater.set(buf, new char[1024]);
            }
            buf.setLength(0);
        }
Reuse {@link StringBuilder} based on {@link ThreadLocal}.

Be careful that do not to nest in the same thread.

@author jiachun.fjc
jraft-rheakv/rheakv-core/src/main/java/com/alipay/sofa/jraft/rhea/util/StringBuilderHelper.java