Message finalzier support

[crazybert]

Finalizer is fired on success and final retry

New interfaces:
IFinalizeMessages
IMessageFinalizerProvider

[judwhite]

Can you explain the use case? We can talk offline this week and I'll update the PR with relevant info.

[judwhite]

The nsqjs library has a DISCARD handler, maybe we could do something like this.

The IMessageAuditor handles these cases but:

• The naming doesn't make it obvious.
• You need to bring your own plumbing if you want to do custom success/final failure handling based on topic.

I kind of get the final failure handling, but what's the use case for success? Is it to support sagas?