A couple of renames in the exposed APIs. MapBasedEventPublicationRepository now uses a a TreeMap again instead of a List (closer to what the original EventPublicationRegistry did).
Cleanups in the JDBC event registry module. Polish pom.xml to minimize dependencies. Removed custom DatabaseType in favor of Boot's DatabaseDriver. Tweaked the auto-configuration of the JDBC module to not even expose a DatabaseSchemaInitializer in the first place to avoid having to wire the boolean flag into the latter.
Cleanups in test cases to avoid code duplication. Move to @JdbcTest to avoid having to depend on Spring Data JDBC.
Enable nullability checks, formatting, Javadoc, author tags, missing license headers.
