Tweak `ClickHouseChain._get_rows` to use of `slc` setting already in the `SELECT` query

Using `slc.start` and `slc.stop` should be comparably simple.
It's not _easy_ because `start` and `stop` can be negative...

Also using `step` in the `SELECT` query might be too difficult to implement in the query.

The tests from `test_utils.py` that were added in #49 should cover it.