Refactor computeMemUsed into a separate class

Currently, freePages() manipulate stats depending on different actions (compact, evict recovery). The logic can be pushed down to individual page actions. These page actions can collect stats at a page level, and the plasma stats will be updated accordingly in updateMapping().

Currently, stats on a page is executed through a single function computeMemUsed(). This function can be made into a independent class that walks down the page delta and collects stats as a result. This can be called from individual page action to collect page level stats.

Unlike page walker, this class will not read records from disk.