private StringBuilder getStringBuilder() {
            truncate();
            return buf;
        }
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