Swarming: fix the exception in tidy_stale
Swarming: fix the exception in tidy_stale

ndb.tasklet doesn't like when synchronous calls are done during an asynchronous
context. It doesn't fail outright, just randomly throws with random unrelated
exception, like:
    File "appengine/runtime/request_environment.py", line 126, in __getitem__
      return self._request.environ[key]
  RuntimeError: maximum recursion depth exceeded in cmp

Lovely, right?

I've read this code so many times, yet failed to see the ONE OBVIOUS
synchronous call left by error. Dang. Fix it.

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Bug: 810008
Change-Id: I84bf02b06aa33f45c32a95abcb74a6b1119e063b
Reviewed-on: https://chromium-review.googlesource.com/940103
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

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