Keep page level statistics up-to-date

Plasma SMO adjustment depends on page level statistics. Currently, the stats is a slow moving average, depending on how long the indexer has been run. In order to make SMO adjustment reflects the current state of the index, page level stats need to be more accurate:
1) update stats only when page is compacted or swap-in
2) include keys in delta chain in the stats
3) update stats when an old page is freed due to compaction