[swarming] add more debug log to _expire_task_tx
[swarming] add more debug log to _expire_task_tx

crrev.com/c/1948019 wasn't sufficient

Bug: 1030504
Change-Id: I75a91e455d3c452385db1d2fcd171f332c3dac97
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1954835
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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