Stats management improvements

Currently CH manages its state by storing it along with other data to "rare" and "often" streams on disk. The states of various objects are stored in a shared file and they don't manage the state by themselves (i.e. no checking if there were any changes done in the state since the last save, they can't decide how to store the information - by storing everything again or only by diff).

We need to address those issues and make sure there is an easy way to introduce other states for different objects.