[milo] improve delete-builds cron job to delete builds faster
[milo] improve delete-builds cron job to delete builds faster

1. Split builds into batches.
2. Delete build batches in parallel.

R=mdraz, nqmtuan

Bug: 981947
Change-Id: I7f5a3dab2bf16ca38366345349c2163004a1e79b
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3551179
Reviewed-by: Tuan Nguyen <7f5d2f4f6ce87649784e35ad551490ab8d89e8eb@google.com>
Commit-Queue: Weiwei Lin <968518a24f41fef00b747498600e52b792d06ae0@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go