Implement XAResource for Session

The JSR-283 specification describes in Chapter 21 how the behavior of Session.save() and other workspace-direct methods are different when operating within the scope of a UserTransaction. This basically means that we'll have to provide an XAResource implementation so that the session can receive notifications of the various commit/rollback stages.

We'll also have to change how events are processed, to not really be fired to the rest of the system (from the connector) until JTA transaction is committed (via the XAResource). This may be challenging because of how the connector framework sets up connectors with session-independent event listeners.