TIL; Symfony messenger & doctrine_transaction
Symfony messenger can be configured to use the doctrine_transaction middleware
in order to encapsulate all SQL queries performed within command handlers in a
single transaction.
As a main consequence, it's no longer necessary to call flush from each
handlers, as it will now be performed by the middleware. Dependency injection is
therefore lightened, as we don't need to inject the entity manager anymore for
that usage.