[milo] Parallellise project datastore updates.
[milo] Parallellise project datastore updates.

Bug: 1068630
Change-Id: I75a025704404d4d9114894b5f89b0701c131dda3
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2280686
Reviewed-by: Weiwei Lin <968518a24f41fef00b747498600e52b792d06ae0@google.com>
Commit-Queue: Joey Scarr <e609d4c46132d2dea94acf5ac2b9f229ffc1f245@chromium.org>

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