Stop queue:work --daemon processing when app down.

Contrary to the documentation, the 'queue:work --daemon' command happily
continues on processing events after the 'down' command has been called,
which could lead to queue events being processed while database and code
migrations make the application unstable.

This fix adds a isDownForMaintenance() function into the QueueManager to
provide the Worker a way to find out if the application is down or not.

Implemented as part of the existing daemonShouldRun() function, since
that function already performs the required process of blocking future
events from being processed.
Merge pull request #5565 from valorin/bugfix-stop-queue-daemon-when-down

Stop queue:work --daemon processing when app down.