Added SQLite adapter

Fixed paginated data merge.

• fixed the docker build
• fixed the database adapters initialisation

Added MongoDB adapter.

Added PostgreSQL adapter.

Added the progress indication. The fetching process is slow with serverless functions, so it's good to understand if it's not stuck and actually loading.

Fixed the clean up function scheduled run. Netlify call started to time out for the same reason as the GET call. I moved the clean up run to be run along with the fetching data call.

Updated the data to be fetched sequentially, so it doesn't use too much memory on the server side. Netlify function started to time out, when more data got aggregated.

Added two new environment variables for better tweaking.

Initial stable release.