Tag runs as lukewarm

[rschu1ze]

This PR addresses #667. It introduces a distinction between lukewarm runs and cold runs. The former is considered legacy and README.md now encourages to migrate to the latter.

For the time being, all existing submissions were labeled lukewarm-cold-runs except

• DBaaS entries (Redshift, BigQuery, etc.),
• the databases touched by Prevent lukewarm cold runs in databases from Munich #659 (Umbra, Hyper, CedarDB) - these have proper cold runs already.
• all entries below "versions/" and "hardware/" (they are unrelated to the official ClickBench benchmark).

We encourage PRs that migrate submissions from lukewarm to cold runs.
(We'll adjust all ClickHouse-related entries by ourselves soon).