boolean loadHeader() {
        int oldPos = this.buffer.position();
        try {
            BufferUtils.position(this.buffer, 0);
            return this.header.decode(this.buffer.asReadOnlyBuffer());
        } finally {
            BufferUtils.position(this.buffer, oldPos);
        }
    }
Clear data in [startPos, startPos+64).

@param startPos the start position(inclusive)
jraft-core/src/main/java/com/alipay/sofa/jraft/storage/log/SegmentFile.java