Implement update_seq and instance_start_time fields for /{db} endpoint

The `KeyValueStoreSortedInterface` should be completely generic and not have anything to do with "sequence ID". The `key_value` module calls this key "score" and it's specific to sorted sets (`KeyValueStoreSortedSetInterface`). So let's rename the method and move it here: `KeyValueStoreSortedSetInterface::getMaxScore()`. For consistency we should also add `KeyValueStoreSortedSetInterface::getMinScore()`.