For certain cases we can now update the cached statistics in
CollectionStatistics incrementally instead of invalidating the
cache and querying DB to calculate it again. The calculation is
rather expensive (one of the most expensive queries we have) and
the time we spent recalculating the stats quickly adds up to secons.

Currently we only support incremental stats update for flags change
within a single collection and for appending new items which,
especially in the second case, should notably reduce the amount of
the super-heavy statistics queries.

In any other case we currently fall-back to invalidating the cache
and calculating the stats on demand.