Partition Skylab jobs by started_time

Introduce an new field partition_timestamp which equals queued_time
whenever it is populated and started_time otherwise.

After this Skylab jobs will start appearing in tko_jobs.

Skylab jobs do not have the field queued_time populated. Therefore,
using queueud_time for table partitioning resulted in Skylab jobs being
omitted.

TEST=populated ci_results_dev.tko_jobs
BUG=b:122081206

Change-Id: I2b7587302792f5d5990daf1ab8015ad4e642492f
Reviewed-on: https://chromium-review.googlesource.com/1416323
Commit-Ready: Alex Zamorzaev <zamorzaev@chromium.org>
Tested-by: Alex Zamorzaev <zamorzaev@chromium.org>
Reviewed-by: Alex Zamorzaev <zamorzaev@chromium.org>
3 files changed