Add hooks to output connectors for receiving event notifications, specifically job start, job end, etc.

Currently there is no logic that informs an output connection of a job start, end, deletion, or other activity. While this would seem to have little to do with an output connector, this feature has been requested by Jack Krupansky as a potential way of deciding when to tell Solr to commit documents, rather than leave it up to Solr's configuration.