Use spawn_now_dangerously for better efficiency.
Revert "Use spawn_now_dangerously for better efficiency."

This reverts commit 08e715e5a503116bca24aa6dde8252855a689579.
Revert "Parallelize backfill chunk inserts (partially)."

This reverts commit 5e1e3231ed9c19b061a4c20f8528ade05354ccc1.