[Messenger] Add a middleware that wraps all handlers in one Doctrine transaction

[javiereguiluz]

symfony/symfony#26647

We'll need help to document this. Hopefully @Nyholm and/or @sroze can help us here. Thanks!

[sroze]

Note that its integration within FrameworkBundle has been reverted in symfony/symfony#26684.

[Nyholm]

I will make the pr and improve the docs.