all: make more use of the new atomic integer types

all: make more use of the new atomic integer types

Slightly simplifies the code and avoids human error.

Change-Id: Ib76575e8bc5b3a699ba6cc3870d63cd7a55e6416
Reviewed-on: https://go-review.googlesource.com/c/go/+/541476
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>