Bulk inserts: Alternative POC

This was broken out of #36992 (closed) after reviewing the first POC in a debrief meeting.

@ayufan mentioned he had an idea about taking a different approach, which would not involve rewriting the callback chain, but instead leverage AR's CollectionAssociation (i.e. the class that models has_many associations), so as to "inject" records we want to bulk insert at this level instead.

Needs more grooming.