bidirectionally links two entries of the LRU linked list
function link(nextEntry, prevEntry) {
        if (nextEntry !== prevEntry) {
          if (nextEntry) nextEntry.p = prevEntry; //p stands for previous, 'prev' didn't minify
          if (prevEntry) prevEntry.n = nextEntry; //n stands for next, 'next' didn't minify
        }
      }