Refactor agent in a way that apps hold some state

Closes #474 (closed)

Apps should be objects with methods. There should be a base app and then Kea and BIND 9 apps that inherits from the base app. They should provide some common interface. They should have specific implementation for common interface and also some extra, specific interface.

This change only introduces base and specific apps and common interface. The next changes will introduce specific interfaces.