public T peekFirst() {
        Segment<T> firstSeg = getFirst();
        return firstSeg == null ? null : firstSeg.peekFirst();
    }
A recyclable segment.
@author boyan(boyan@antfin.com)

@param <T>
jraft-core/src/main/java/com/alipay/sofa/jraft/util/SegmentList.java