isolate: use eventual consistency in slow query
isolate: use eventual consistency in slow query

Currently, query in cleanup cron sometimes becomes timeout.
http://shortn/_YKixShRRIN

For faster query execution, let me try to use eventual consistency
option.
https://cloud.google.com/appengine/docs/standard/python/ndb/queryclass

Bug: 1004389
Change-Id: If955a34b5aa9886a18151d396ce05a6ac3b79bc9
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1916991
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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