swarming: reduce cache trashing in most cron and task queues
swarming: reduce cache trashing in most cron and task queues

Many churn through many TaskRequest's and other entities. The amount of memory used
by the local cache may become excessive, especially that these are one time fetch,
causing AppEngine 'Exceeded soft private memory limit of 512 MB...', which was
observed on prod and is just noise.

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org
Bug:
Change-Id: I74862c657e1daa822639c14e78c563bbfec6bd38
Reviewed-on: https://chromium-review.googlesource.com/764656
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