Preserve last_updated for MapBuilder

[shyamd]

Add a feature to preserve the last_updated data in MapBuilder.

This is usefull for derived data collections where we lazily just recompute and update everytime we run that builder rather than only updating new things.

[dwinston]

What do you mean by “preserve”? MapBuilder normally doesn’t update the last_updated field of the target store.

[shyamd]

You're right. T'was a different builder that was the offender.