Clear page cache in some scripts

[rschu1ze]

Resolves #774

(Same as #786 but without destroying my local Git branches)

From the list in #774

• clickhouse-datalake and clickhouse-datalake-partitioned: ✅
• duckdb-dataframe: ✅
• elasticsearch: the script broke in the meantime, need to fix separately (only added page cache clearing in this PR) ☑️
• hydra: DBaaS, didn't adjust the script ☑️
• locustdb: completely broken, dead upstream ☑️
• mongodb: benchmark script is in JavaScript, not clear how to clear page cache from there ❓
• pandas ✅
• polars ✅
• polars-dataframe ✅
• tembo-olap: DBaaS, didn't adjust the script ☑️

This PR also removes

# On small machines it can only work with swap
sudo fallocate -l 200G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

from some submissions to make benchmarking overall more fair.