makes the `entry` the freshEnd of the LRU linked list
function refresh(entry) {
        if (entry !== freshEnd) {
          if (!staleEnd) {
            staleEnd = entry;
          } else if (staleEnd === entry) {
            staleEnd = entry.n;
          }

          link(entry.n, entry.p);
          link(entry, freshEnd);
          freshEnd = entry;
          freshEnd.n = null;
        }
      }