Protect against inconsistent index.
Protect against inconsistent index.

- yield_run_result_keys_with_dead_bot() in task_result.py yields an eventually
  consistent index query.
- cron_handle_bot_died() in task_scheduler.py assumed that the index is
  consistent with regards of modified_ts; In particular _handle_dead_bot()
  didn't confirm that modified_ts was stale enough before killing a task.

Fix: Add explicit check that modified_ts is stale enough to warrant killing the
task at the begining of the transaction.

Bug found via ereporter2 log analysis. This bug is suspected to be the major
source of remaining internal_failure.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1508453003 .

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