Don't instantiate CoreSession through service adapter

Today to acquire a LocalSession (CoreSession implementation) we do (in CoreInstance): Framework.getService(CoreSession.class) which uses the RepositoryService.getAdapter method.

This is a problem when using recent code in nuxeo-apidoc-server in org.nuxeo.apidoc.introspection.ServerInfo which does:

    protected static boolean isServiceOverriden(RegistrationInfo ri, String serviceName) {
        Class<?> typeof = Class.forName(serviceName);
        final Object adapter = ri.getComponent().getAdapter(typeof);
        final Object service = Framework.getService(typeof);
        if (adapter == service) {
            return false;
        }
        return service.getClass() != adapter.getClass();

So just checking the adapter actually instantiates a CoreSession, which is bad (and it's not closed).

Instead to acquire the CoreSession we could simply call an explicit method on the RepositoryService.