Expose mutation count for index keys - for MOI storage

In Cheshire-Cat, CBO relies on statistics gathered through UPDATE STATISTICS command for proper cost calculations. The responsibility for running UPDATE STATISTICS is on the user. The query team is considering Auto-Update-Statistics as a new enhancement for next release. Basically the query engine will keep track of and monitor changes to the index keys, and once the mutation count exceeds a certain threshold, a new UPDATE STATISTICS command will be run in the background to refresh index and distribution statistics.