insights: track backfill attempts

Created by: coury-clark

It will be useful for the insights backfiller process to record how many attempts were made to perform the complete backfill. This is a lightweight solution to maintaining some state about the progress of the backfill. In the short term this will be used to silence scoped series that are failing too many times.

Proposal:

1. Add a column to insight_series called backfill_attempts
2. Each time a series is loaded for backfill increment backfill_attempts. Note: this should be done prior to attempting the backfill so that any transients failures mid-process don't lose count