Add retry support for serializable transactions (#386)

## tl;dr

I noticed our `libxmtp` test suite was occasionally failing due to serializable transaction errors. This adds retries, which are required when using the serializable isolation level since errors are common and transient thanks to Postgres' conflict detection

pkg/mls/store/store.go

@@ -106,7 +106,7 @@ func (s *Store) PublishIdentityUpdate(ctx context.Context, req *identity.Publish
 		return nil, errors.New("IdentityUpdate is required")
 	}
 
-	if err := s.RunInTx(ctx, &sql.TxOptions{Isolation: sql.LevelSerializable}, func(ctx context.Context, txQueries *queries.Queries) error {
+	if err := s.RunInSerializableTx(ctx, 3, func(ctx context.Context, txQueries *queries.Queries) error {
 		inboxLogEntries, err := txQueries.GetAllInboxLogs(ctx, new_update.GetInboxId())
 		if err != nil {
 			return err
@@ -605,3 +605,20 @@ func (s *Store) RunInTx(
 	done = true
 	return tx.Commit()
 }
+
+func (s *Store) RunInSerializableTx(ctx context.Context, numRetries int, fn func(ctx context.Context, txQueries *queries.Queries) error) error {
+	var err error
+	for i := 0; i < numRetries; i++ {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
+			err = s.RunInTx(ctx, &sql.TxOptions{Isolation: sql.LevelSerializable}, fn)
+			if err == nil {
+				return nil
+			}
+			s.log.Warn("Error in serializable tx", zap.Error(err))
+		}
+	}
+	return err
+}