Switch to external stringset implementation.

impl/memory/datastore_index.go

Index: impl/memory/datastore_index.go
diff --git a/impl/memory/datastore_index.go b/impl/memory/datastore_index.go
index f4db119e6bd8af872caa037749419d62181a2e38..f3c12d1b5523f60c333d55e5e57bb6c1b134ed0b 100644
--- a/impl/memory/datastore_index.go
+++ b/impl/memory/datastore_index.go
@@ -12,6 +12,7 @@ import (
 	ds "github.com/luci/gae/service/datastore"
 	"github.com/luci/gae/service/datastore/serialize"
 	"github.com/luci/gkvlite"
+	"github.com/luci/luci-go/common/stringset"
 )
 
 type qIndexSlice []*ds.IndexDefinition
@@ -68,7 +69,7 @@ func partiallySerialize(k ds.Key, pm ds.PropertyMap) (ret serializedIndexablePma
 		k = k.Parent()
 	}
 	for k, vals := range pm {
-		dups := stringSet{}
+		dups := stringset.New(0)
 		newVals := make(serializedPvals, 0, len(vals))
 		for _, v := range vals {
 			if v.IndexSetting() == ds.NoIndex {
@@ -76,7 +77,7 @@ func partiallySerialize(k ds.Key, pm ds.PropertyMap) (ret serializedIndexablePma
 			}
 			data := serialize.ToBytes(v)
 			dataS := string(data)
-			if !dups.add(dataS) {
+			if !dups.Add(dataS) {
 				continue
 			}
 			newVals = append(newVals, data)