public PubSubPersistenceProvider getProvider() {
        return this.provider;
    }
Instantiates a new instance.

It is not expected that there ever is more than one instance of the manager, as it's closely coupled to
{@link PubSubModule}. This constructor has 'package' access restrictions to reflect this.
xmppserver/src/main/java/org/jivesoftware/openfire/pubsub/PubSubPersistenceProviderManager.java