Reland "swarming: re-get entity when completed_ts is not set"
Reland "swarming: re-get entity when completed_ts is not set"

This is a reland of 695ab627ba81ccc16c0fab07954c08d9b4784104

I guess the reason of Deadlock is yet another RuntimeError of recursion
limit exceeded.
https://screenshot.googleplex.com/Nxb7EMfWLPY

Let me use ndb.Future.wait_all, this may avoid large depth recursion.

Original change's description:
> swarming: re-get entity when completed_ts is not set
>
> Maybe getting entity by using key can fix the issue?
>
> Bug: 1064833
> Change-Id: If2ad7d4f72efbd64f40944fb2432a3874f62381a
> Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2134333
> Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
> Reviewed-by: Ye Kuang <yekuang@google.com>
> Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

Bug: 1064833
Change-Id: Ic4d0a23bfe399403ad5c1c1450c5a76a298a4126
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2137180
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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