cas: use SetMulti for batch write to cache
cas: use SetMulti for batch write to cache

We cannot use sync.Pool due to asynchronous buffer write to db.

downloading input for blink_web_tests with badger
https://chromium-swarm.appspot.com/task?id=523e42523214af10
no   cache: 76.5439999104
full cache: 63.4010000229

downloading input for blink_web_tests with bbolt
https://chromium-swarm.appspot.com/task?id=523dda7751095a10
no   cache: 89.006000042
full cache: 70.876999855

input is taken from
https://chromium-swarm.appspot.com/task?id=520b3928230ba210

Bug: 1172879
Change-Id: Id1932c5353f6976688a28f40585c4108cfe7f075
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2755352
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <5d95c5c75e3d691d9c77888fff4498e04f8b7157@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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