cv: maximize the rate for update-cl task queue
cv: maximize the rate for update-cl task queue

update-cl queue should be drained as soon as possible to make sure CV
react to user's CQ vote in a timely fashion. Even though, the task may
not need the maximum rate at this time point, it is better to set it
to maximum number so that when outage happens, the task queue can
recover quicker.

R=ddoman, robertocn

Bug: 1328712
Change-Id: I3f1d4cdbfbf9e7d6570ef14762d51df0eb65c309
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3664796
Reviewed-by: Roberto Carrillo <b31161e510fbb37f8f70fdcde11530d2bae88470@chromium.org>
Auto-Submit: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Commit-Queue: Roberto Carrillo <b31161e510fbb37f8f70fdcde11530d2bae88470@chromium.org>
Reviewed-by: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go