syncmap: release m.mu during (*RWMutexMap).Range callbacks

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

Change-Id: Id73d254fa01cc64a1f00eb1903488796e1282423
Reviewed-on: https://go-review.googlesource.com/42956
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>