Audit backends used as storages should actually be instanciated as backends

In order to be able to migrate log entries from Elasticsearch to MongoDB, we need to be able to instanciate multiple audit backends. The current issue is that ESAuditBackend is also the default ES audit storage, and we can only declare one audit backend.
A solution would be to check the audit storage classes when NXAuditEventsService starts, and if it is actually an audit backend, instanciate it as such (an audit backend registry would probably be necessary).