Method new_input_iterator

Create an iterator that reads over all the compaction input tables with merged order. We produce different iter for tables in level0 and level >0 : level 0: Since key ranges might be overlapped with each other, we generate a table iterator over every single level 0 sst file level > 0: a `ConcatenateIterator` for all the sst file in this level Entry format: key: internal key value: value of user

(
        &self,
        icmp: InternalKeyComparator<C>,
        table_cache: TableCache<S, C>,
    )