swarm/chunk: add tags data type

* swarm/chunk: add tags backend to chunk package

swarm/sctx/sctx.go

@@ -5,12 +5,15 @@ import "context"
 type (
 	HTTPRequestIDKey struct{}
 	requestHostKey   struct{}
+	tagKey           struct{}
 )
 
+// SetHost sets the http request host in the context
 func SetHost(ctx context.Context, domain string) context.Context {
 	return context.WithValue(ctx, requestHostKey{}, domain)
 }
 
+// GetHost gets the request host from the context
 func GetHost(ctx context.Context) string {
 	v, ok := ctx.Value(requestHostKey{}).(string)
 	if ok {
@@ -18,3 +21,17 @@ func GetHost(ctx context.Context) string {
 	}
 	return ""
 }
+
+// SetTag sets the tag unique identifier in the context
+func SetTag(ctx context.Context, tagId uint32) context.Context {
+	return context.WithValue(ctx, tagKey{}, tagId)
+}
+
+// GetTag gets the tag unique identifier from the context
+func GetTag(ctx context.Context) uint32 {
+	v, ok := ctx.Value(tagKey{}).(uint32)
+	if ok {
+		return v
+	}
+	return 0
+}