multi: add reset closure to kvdb.Update

Similarly as with kvdb.View this commits adds a reset closure to the
kvdb.Update call in order to be able to reset external state if the
underlying db backend needs to retry the transaction.

discovery/message_store_test.go

@@ -239,7 +239,7 @@ func TestMessageStoreUnsupportedMessage(t *testing.T) {
 	err = kvdb.Update(msgStore.db, func(tx kvdb.RwTx) error {
 		messageStore := tx.ReadWriteBucket(messageStoreBucket)
 		return messageStore.Put(msgKey, rawMsg.Bytes())
-	})
+	}, func() {})
 	if err != nil {
 		t.Fatalf("unable to add unsupported message to store: %v", err)
 	}