The mainline sync.Map has allowed mutations within Range callbacks
since https://golang.org/cl/37342. The reference implementations need
to do the same.

This change integrates https://go-review.googlesource.com/c/42956/
from x/sync.

Change-Id: I6b58cf874bb31cd4f6fdb8bfa8278888ed617a5a
Reviewed-on: https://go-review.googlesource.com/42957
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>