protected byte[] getKeyBytes(final long index) {
        final byte[] ks = new byte[8];
        Bits.putLong(ks, 0, index);
        return ks;
    }
First log index and last log index key in configuration column family.
jraft-extension/bdb-log-storage-impl/src/main/java/com/alipay/sofa/jraft/storage/impl/BDBLogStorage.java