introduce sync calls for systems not doing fsync

[lukasvogel]

Added sync around data loading for ClickHouse and Ursa as their load times are below the theoretical minimum for instances where the whole dataset fits into memory (cf Issue #678) .

I would also have liked to done it for TCHouse-C and Clickhouse Cloud but I have no idea of how to do that there.

I'm also sure there are other systems that don't do syncs but have longer load times anyway - I currently don't have time to test them all, though.

Also measured that locally on CedarDB and DuckDB - didn't have any effect, as expected. If you want to, we can also include it in future commits, though!