CBD-821 Adapt the item pager to use the extended NRU bits.

An item's NRU value is extended to two bits, which allows us
to have the range of 0 - 3 values. More frequently accessed
items will likely have zero NRU value as their NRU values are
decremented everytime they are referenced.

The item pager is adapted to leverage the extended NRU bits:

Phase 1:
1. Scan the next vbucket and collect items whose NRU score is 3
2. Evict those items with NRU score '3'
3. Check the memory usage and go to step 1 if the memory usage
   is still above the low watermark.

Once the phase 1 is completed, then the pager goes to the phase
2 if the memory usage is still above the high watermark.

Phase 2:
1. Scan the next vbucket and increment every item's
   NRU score by 1
2. If its NRU score is equal to '3', then generate a random
   number N whose range is from 0 to 1, and evict that item
   if N < P where P is the probability that is calculated based
   on the current memory usage and memory low watermark.
   Note that we also differentiate the probability P depending on
   the vbucket state (active, replica).
3. Check the memory usage and go to step 1 if the memory usage is
   still above the low watermark.

Change-Id: Idfcfda834e5fe89061cd49d5abaefba0621c6a42
Reviewed-on: http://review.couchbase.org/24526
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>