title: MemoryCache with certain number of items
body: <p>does MemoryCache has functionality to cache fixed number of items?</p>
  
  <p>e.g. We are only interested in cache 2000 items from database. While keep adding items to the cache, if the specified number of items are exceeded, the oldest one can be removed.</p>
  
  <p>If not, do we have to use another thread to do the house keeping regularly? </p>