Unique Message Controls

[ChuckJonas]

Add ability to control message uniqueness. There are two, somewhat different use cases where some "uniqueness" controls might be desired:

1. Only allowing a specific message to be queued once (must be considered with message TTL)
This is about preventing an actual event from occurring twice. Typically would be implemented by assigning a "Message Id" and enforcing uniqueness constraint to prevent duplicates.

2. Not creating a duplicate outbox if there is an equivalent one with an active status:
The is more about ensuring efficiency by not queueing up a message that is an effective duplicate of one that is already in the queue. The can be a real "footgun" in temporal sensitive scenerio's and might be hard to implement.