swarming: Reduce parallelism to reduce runtime errors in tidy_stale
swarming: Reduce parallelism to reduce runtime errors in tidy_stale

Using 100 batch size for two concurrent queries meant that hundreds of
transactions could happen concurrently,  via _remove_old_entity().

Drastically limit to two batches of 16, which should keep the number of
concurrent RPCs below the effective limit.

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Bug:
Change-Id: Ibfed56a399748a5b7244f1c37972c0cde30767ab
Reviewed-on: https://chromium-review.googlesource.com/765263
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