[swarming] Tweak Swarming backend module performance
[swarming] Tweak Swarming backend module performance

- Reduce backend instance cost by 50% This will lower costs from 30¢/h to 20¢/h.
- Increases the acceptable latency, to reduce the observed instances
  churn. This is important because each created instance has a 15 minimum
  charge.

Bug: 939204
Change-Id: I2abcbc7a20fdfcbf2ee2acb59904e14dbe978283
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1509338
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>
Auto-Submit: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py