🚧 change jdbc backend logic

[tyrasd]

Description

• cells are fetched individually instead of in a single sql query
• properly closes database resources

Corresponding issue

Was split off from #419, see https://github.com/GIScience/oshdb/pull/419#discussion_r688599135

Todo

• [ ] benchmark changes
• [ ] change target branch to master after #419 is merged
• [ ] for this to work properly OSHDBJdbc should not return a single share connection but a pool of connections

Checklist

• [ ] My code follows the code-style rules, and I have checked on the static analyses and benchmark (if applicable) results
• [ ] I have commented my code
• [ ] I have written javadoc (required for public classes and methods)
• [ ] I have added sufficient unit tests
• [ ] I have made corresponding changes to the documentation
• [ ] I have updated the CHANGELOG.md
• [ ] I have adjusted the examples or created an issue in the corresponding repository
• [ ] I have adjusted the benchmark or created an issue in the corresponding repository