Functions usage statistics by period

Add statistics for function usage by period with filtering, available at endpoint GET /stat/user
Changed URL of overall stats from /stat to /stat/total

Node roles

Introduced node roles: calculation, projection, api and lighthouse
Changed system design, dedicated node to api, cluster discovery (lighthouse) and domain logic (calculation + projection)
Persistence is considered stable
Done a lot of work to run integration tests on k8s environment inside CI\CD
New Readme description

Persistence preview

Preview release with calculation statistics functionality