[cv][tryjob] actually use RWMutex.
[cv][tryjob] actually use RWMutex.

While playing with RW vs normal mutex performance, I forgot
to upload this to the prior CL.

R=ddoman, yiwzhang

Change-Id: I3404838bdf07fa10382ab0c5bd8942b751fd3b2a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3293403
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

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