protected void onTruncatePrefix(final long startIndex, final long firstIndexKept) throws RocksDBException,
    IOException {
    }
Called after truncating prefix logs in rocksdb.

@param startIndex     the start index
@param firstIndexKept the first index to kept
jraft-core/src/main/java/com/alipay/sofa/jraft/storage/impl/RocksDBLogStorage.java