Fix kernel events handling in maintenance/update mode

[cedric-anne]

Checklist before requesting a review

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.

Description

The CheckIfUpdateNeededListener and CheckMaintenanceListener were defining the controller in the route attribute, but this does not stops the event propagation automatically. It means that subsequent listeners were still evaluated, and it may result in executing unexpected code.

To prevent this, I refactored the listeners to add some more checks to prevent redefining a controller defined by a previous listener.

[cedric-anne]

I figured out that the Symfony profiler bar was trigerring 404 errors on the maintenance page. After a quick investigation, I figured out that some request listerners of the Symfony framework bundle were not execued int his case, and it was causing the issue.
To be sure that Symfony listerners are correctly executed, I had to refactored a bit the code, see 2nd commit.