all: use time.Since instead of time.Now().Sub

all: use time.Since instead of time.Now().Sub

Change-Id: Ifaa73b64e5b6a1d37c753e2440b642478d7dfbce
Reviewed-on: https://go-review.googlesource.com/c/go/+/436957
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: hopehook <hopehook@golangcn.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>