Coalesce Subscriptions per callback URL

[BenjaminPelletier]

Currently, we report a Subscriber for each Subscription. Instead, we should coalesce per URL so that each unique URL only produces one SubscriberToNotify.

Is it correct to assume that there is always only one subscription ?

Originally posted by @barroco in #784 (comment)

[BenjaminPelletier]

There may be many Subscriptions with the same callback URL, but that callback URL only needs to be invoked once. The SubscriberToNotify structure contains a list of Subscriptions that are triggering this (one) notification.