add event for recordkeeper is updating stockpile records
Several plugins (e.g. stockflow, tailor) currently have code that detects when the recordkeeper starts updating the fortress's stockpile records and use this to trigger the effects of these plugins.
This code is duplicated and seems to fit within the scope of eventful; thus, it seems reasonable for "recordkeeper begins updating stockpile records" to be an event that other plugins/scripts can capture.
I'm going to bump this issue because I'm not sure that the code in tailor that is supposed to implement this is working correctly (for various reasons), which means I'd like a more reliable solution than what's being used now which would I think would ideally be done through eventful.