sync: switch Map to use atomic.Pointer
sync: switch Map to use atomic.Pointer

There was no noticeable change in performance.

Change-Id: I9c57bf836c8b6066e0620afb3d536ce99e4b9d87
Reviewed-on: https://go-review.googlesource.com/c/go/+/426074
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Auto-Submit: Joseph Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@digital-static.net>
Run-TryBot: Joseph Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@digital-static.net>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go