[swarming] Set deadline to _reap_task() Datastore transaction.
[swarming] Set deadline to _reap_task() Datastore transaction.

Set deadline=30 for _reap_task transaction.
https://cloud.google.com/appengine/docs/standard/python/ndb/functions#context_options

This is to avoid AppEngine 1 minute request timeout during Datastore transaction.

Bug: 1274442
Change-Id: I3955288e215be01e11a5b83ab0e6fbc02a3a0309
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/3304909
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