swarming: increase batch_size when looking at expired tasks
swarming: increase batch_size when looking at expired tasks

Trace noted that the default value of 20 is really too low.

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Bug:
Change-Id: I077cb62f86d66326cbff55152e72c19c636864ee
Reviewed-on: https://chromium-review.googlesource.com/772323
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