[buildbucket] decrease num of parallel goroutines in monitor
[buildbucket] decrease num of parallel goroutines in monitor

(256, 64) seems to show the best performance with the current dataset.

Change-Id: Ied1ba95c23a1f0ad5f9ed91d766635a9206304e5
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3362251
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>

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