Viktor/backend

[zilopbg]

This MR introduces the following changes:

1. Upgrades gradle to 5.6.4, the sqlite library to 3.45.2 and sets the Java version to 11.
2. Utilizes a simple DAO pattern to handle access to the different data sources.
3. Adds the necessary logic to aggregate data (DefaultPopulationAggregator).
4. Creates a class (DefaultCountryNameResolver) that should handle countries that exist under different names in the data-sources.
5. Adopts log4j2 for logging.
6. Adds unit tests.
7. Aims to improve the code quality by introducing several packages, interfaces, etc. All classes receive the required dependencies through their constructors, so Spring could be adopted easily if/when needed.
8. Creates a Solution.md file with instructions how to run the code.

Several TODOs are added in the code as points to discuss.