avoid deadlock in syncMap.Iterate (#909)

Previously, when syncMap iterates a list of records which have the same
content in different order, a deadlock might happen. By enforcing a certain
order, the deadlock can be avoided.

sync/map.go

@@ -5,6 +5,7 @@ import (
 	"encoding/base64"
 	"encoding/json"
 	"fmt"
+	"sort"
 
 	"github.com/micro/go-micro/store"
 	ckv "github.com/micro/go-micro/store/etcd"
@@ -94,6 +95,10 @@ func (m *syncMap) Iterate(fn func(key, val interface{}) error) error {
 		return err
 	}
 
+	sort.Slice(keyvals, func(i, j int) bool {
+		return keyvals[i].Key < keyvals[j].Key
+	})
+
 	for _, keyval := range keyvals {
 		// lock
 		if err := m.opts.Lock.Acquire(keyval.Key); err != nil {