This finally adopts the new standardized interfaces instead of the
work-in-progress ones with the `Interop\` prefix.
Since we have now updated to PHP 7.1, we can also use Stratigility
3.0 as the middleware dispatcher.
With this change, session objects are no longer instantiated
globally, but instead created within a middleware during the
request lifecycle.
In addition, session garbage collection is integrated with
the already existing middleware for this purpose.
This prevents garbage collection to randomly break the installer:
before installation, the models that are being accessed have no
database connection.
Now, the middleware is only mounted into the forum's middleware
stack. I want API requests to have stable performance, and the
forum middleware stack is only mounted when Flarum is installed.
