Continue using timer.lock to simplify timer operations.
[This is one CL in a refactoring stack making very small changes
in each step, so that any subtle bugs that we miss can be more
easily pinpointed to a small change.]
Change-Id: I504335a010d6eb4d7d627145b64a896582158406
Reviewed-on: https://go-review.googlesource.com/c/go/+/564129
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@google.com>