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>

syncmap/map_reference_test.go

@@ -64,8 +64,17 @@ func (m *RWMutexMap) Delete(key interface{}) {
 
 func (m *RWMutexMap) Range(f func(key, value interface{}) (shouldContinue bool)) {
 	m.mu.RLock()
-	defer m.mu.RUnlock()
-	for k, v := range m.dirty {
+	keys := make([]interface{}, 0, len(m.dirty))
+	for k := range m.dirty {
+		keys = append(keys, k)
+	}
+	m.mu.RUnlock()
+
+	for _, k := range keys {
+		v, ok := m.Load(k)
+		if !ok {
+			continue
+		}
 		if !f(k, v) {
 			break
 		}