Extend jcr event model to support sequencing

Since sequencing is an asynchronous operation, we need to be able to tell when a sequencing operation has finished. ATM, we keep trying to query for nodes, for a given amount of time and hope the sequencing output will appear.
We should extend the jcr event model, with a NODE_SEQUENCED event type which gets fired after the session.save (tx commit ?) of a sequencing operation.

This would allow external clients to interact more cleanly with the sequencing process.