Repository for MySQL 8.0 using NOWAIT (and possibly PostgreSQL)

[swistak35]

• [x] make Repository possible to be build without database_url, for producer purposes
• [x] add Repository#insert_record or similar to use it in producers
• [x] use Repository#insert_record in SidekiqProducer
• [x] make SidekiqProducer accept a specific repository, don't hardcode into current one
• [x] make SidekiqScheduler accept a specific repository, don't hardcode into current one
• [x] rename current Repository into Repositories::Mysql57
• [x] drop legacy sidekiq scheduler which is only an obstacle
• [ ] add Lock PORO which serves as common result object for different repositories
• [ ] add Record PORO which serves as common result object for different repositories
• [ ] add Repositories::Nowait

[mostlyobvious]

https://dev.mysql.com/blog-archive/mysql-8-0-1-using-skip-locked-and-nowait-to-handle-hot-rows/