Retain journal entries for valid checkpoints

OAK-4528 will use the journal to answer diff calls. While the default max age for the journal GC is 6 hours, it may still happen that journal entries are deleted when they are actually still needed. E.g. consider a repository with async index update configured and the repository is shutdown with a valid checkpoint for the current async index state. If the repository is stopped for more than 6 hours, the journal GC may rather quickly remove all entries even though the async index update still needs some of them.