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.