[cv][gerrit] decrease noAccessGraceDuration.
[cv][gerrit] decrease noAccessGraceDuration.

Since Gerrit mirror hopping seems to work reasonably well, we can check
other mirrors before storing the CL as likely to be deleted/inaccessble
to CV, which can now be done with higher confidence.

Thus, the duration of uncertainty can be decreased.

R=yiwzhang

Bug: 1220461
Change-Id: I5ebd2664e35a782231613b62858e572e0fd6d018
Fixed: 1220461
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3024295
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

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