Don't allcoate a new math/rand every request.
Don't allcoate a new math/rand every request.

Use a global math/rand instance, rather than re-allocating one for each
request.

BUG=chromium:675485
TEST=None
R=vadimsh@chromium.org

Review-Url: https://codereview.chromium.org/2594463002

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