When my site gets a lot of traffic, I will get a lot of errors like this

request.CRITICAL: Uncaught PHP Exception PDOException: "SQLSTATE[HY000]: General error: 1205 Lock wait timeout exceeded; try restarting transaction" at /var/www/symfony/vendor/symfony/symfony/src/Symfony/Component/HttpFoundation/Session/Storage/Handler/PdoSessionHandler.php line 336

The problem is that I have the lock_mode set to 0 so there shouldn't be any transactions involved in my sessions.