This change protects fields mutated by Trace methods with a mutex.
In particular it fixes races caused by concurrent calls to IsError,
and viewing active traces in /debug/requests.

Includes changes from CL/67730.

Updates golang/go#24231

Change-Id: I07db54c2642efe0ea0e3ffeb54ec796ae51e70ef
Reviewed-on: https://go-review.googlesource.com/98535
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>