Make datbase objects AutoClosable

[redcatbear]

Situation

In cases where you create transient database objects in test cases, you want to make sure that the are dropped after the test case.
The cleanest way to get rid of test-case-specific objects would be to initialize them in a try-with-resources and let Java clean them up. This also works if the test fails.

Acceptance Criteria

• Database objects that support drop() also implement AutoClosable

[redcatbear]

After prototyping this idea, it turns out, it has the side effect of requiring users to use close() in cases where they don't use try-with-resources. This would make it necessary to wrap drop() in close() instead of the other way round and that not only breaks the existing interface, it is also awkward to read in the code.