Dont shutdown before backfill is complete
There was a problem where we could shutdown before issuing the final backfill. Solve by changing two pieces:
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From the backfiller, don't shutdown if there are pending backfill requests. Only check if shutdown was requested if there are no pending backfills
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From the orchestrator, don't tell the backfiller to shutdown until after all reindex operations have completed. The ensures that no backfill requests can be added after shutdown is issued.
Bug: T372128