Optionally store Events in Collector.

This will store the raw event in the Events table if enabled.

There are a few caveats:
1. It seems somewhere in the rpc layer is eating any exception
   being raised from the notification handler, so the ack() handling
   code isn't being reached. This will need to be fixed in oslo.

2. notification delivery_info is being stripped from the notification
   before the handler is called. This means we can't get any routing
   info on the notification (which queue did it come in on, for example).
   Again, this will need to be fixed in oslo.

3. The raw json of the event is not currently stored. The model will need
   to be extended to support this. Next.

Change-Id: Id4687e075e04278d1db6e8acc805c3fed2bd07bb