Comparison mode in performance test.

[alexey-milovidov]

For loop type of queries, execute them on random server and collect statistics (similar to clickhouse-benchmark). We will use statistical test with configurable confidence as an additional stop criteria.

For once type of queries - simly execute them single time on every server.

Please check that statistical criteria is viable - if we test the server on the same machine against itself - it should not find any difference.

[alexey-milovidov]

Implemented and works in CI.