Revert "[swarming] Use a slightly les hackish sleep"
Revert "[swarming] Use a slightly les hackish sleep"

This reverts commit 9fa3b2089ce50cb6fadf0a17c13d50f0e40656e0.

Reason for revert: this fails on prod with:
  unhashable type: 'list'

I'll redo with a unit test to make sure this passes properly.

Original change's description:
> [swarming] Use a slightly les hackish sleep
> 
> Use standard ndb.Future.wait_any() and a list instead of a set of
> ndb.Future. This should help with the deadlock observed on prod. Add a
> log entry to confirm that the log is happening before or after, to see
> if it helped or not.
> 
> Bug: 900042
> Change-Id: I264e6002bfdfcf28e23611cfe5902fb839e5979d
> Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1534883
> Commit-Queue: Jao-ke Chin-Lee <jchinlee@chromium.org>
> Auto-Submit: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
> Reviewed-by: Jao-ke Chin-Lee <jchinlee@chromium.org>

TBR=d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org,jchinlee@chromium.org

Change-Id: Ib24707b6e8a990568f15d231614b859d44758ff9
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 900042
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1534895
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

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