[swarming] add TaskRequest to _request_cache to avoid cross-group transaction error
[swarming] add TaskRequest to _request_cache to avoid cross-group transaction error

https://console.cloud.google.com/errors/CLiKtuDHw8P5sQE?time=PT1H&refresh=auto&project=chromeos-swarming
> BadRequestError: cross-group transaction need to be explicitly specified, see TransactionOptions.Builder.withXG

This error comes from getting parent TaskRequest entity in TaskResultSummary/TaskRunResult.
http://screen/5RDZFwXLs4ewJXy

The parent TaskRequest entity has already been fetched before the transaction.
This CL fill `_request_cache` field with the parent entity to avoid DS access in the transaction.

Bug: 1183873
Change-Id: Ie583b3cbb9ab43d243f96cee0168c0d3050df905
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/3128413
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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