docs: private void purgeTombstones() {
      long minTombstoneTime = membershipService.getMembers()
          .stream()
          .map(node -> updateTimes.getOrDefault(node.id(), 0L))
          .reduce(Math::min)
          .orElse(0L);
      for (InternalTopic topic : topics.values()) {
        topic.purgeTombstones(minTombstoneTime);
      }
    }