Insights: prevent repeated and unnecessary backfills

Created by: coury-clark

Closes #23642 (closed) Closes #22324 (closed)

Introduce a simple state mechanism to prevent series from being backfilled multiple times unnecessarily.

Also rolling back the changes to the sample time to return to the interval time instead of commit time. This turned out to be far too noisy, and we get lots of value from times being aligned.