[swarming] use keys_only query to fetch dead bots
[swarming] use keys_only query to fetch dead bots

Bots.yield_dead_bots() seems failing for datastore operation time out.
http://screen/3gDGgYr6LvAzNJM

This CL
- changes the query to return only keys.
- move task_queues.cleanup_after_bot() to post transaction handler.

If this fix isn't sufficient, will consider using `fetch_page()` to fetch keys step by step.

Bug: 1174290
Change-Id: I953417ad630f57f5ba692fc0fec422f5b69aa235
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2672986
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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