obshashrange: force reset if many changes If there is many new markers (or the markers affect many changesets) resetting affected changeset can get slow. A common case for that is then changeset and markers are added in different transaction, (eg: when using clone bundle). When we detect such case, we reset the obshashrange cache, rewarming it should be faster. Here many is arbitrarily set to 10 000. The number might get adjusted.