This feature is lost when clio was implemented. We may need to port it back from 1.8.

There will be a lot of changes for this since we need to deal with it under cl_page infrastructure, also it might be better to implement this in obdclass/ instead of llite as what 1.8 does.

this is what we should do:
1. Implement generic cl_page LRU mechanism in obdclass/. This LRU list should not use spinlocks for smp scalability and be able to deal with async_page_max cap.
2. add a shrinker for cl_page in llite - not sure if we still need this.