@Override
    public String getOldestTombstone() {
      String result = null;
      if (tombstones.peek() != null) {
        result = tombstones.peek().toString();
      }
      return result;
    }
Returns true if keys needs to be tracked for clients registering interests on PR.
geode-core/src/main/java/org/apache/geode/internal/cache/TombstoneService.java