NServiceBus subscription storage

In a production deployment, publishers persist their subscriber information for two reasons:

1. A scaled-out logical publisher (say, a load-balanced web service that needs to publish events) consists of multiple physical publishers, which all need to have the same subscriber list.

2. If a physical publisher goes down and comes back up, it needs to be able to rehydrate its subscriber list without the subscribers needing to re-send their initialization messages.

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