Mutex for actualize script (#5235)

* Simplify extension method
One parameter was not used

Furthermore, that unused parameter was preventing simpler calls (i.e. without a dedicated extension class). Now it is possible to register a call back with a lamda:

```php
Minz_ExtensionManager::addHook('feed_before_actualize', function () {
	// Hello
});
```

* Mutex for actualize script
In the case of extremely long cron jobs that are refreshing feeds, a cron job might start before the previous one has completed, leading to an accumulation of cron jobs running in parallel.
Although we already have a mutex preventing concurrency problems on a single feed refresh, the problem occurs in particular if each feed can take a long time (e.g. due to heavy processing, which is my usecase).
https://github.com/FreshRSS/FreshRSS/pull/5234 must be merged first

* Fiw wrong return

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