Add Phoenix.PubSub.subscribe_once/3

[nathanl]

Given that the possibility of duplicate subscriptions was already discussed in the docs on subscribe/3, I assume there's a performance cost to checking for existing subscriptions that callers may not always want to pay. If so, I'd be happy to document that on subscribe_once/3. But it seems useful to have this function; I'm looking at a use case where the number of subscriptions to a topic won't be large but duplicate messages would be problematic.

Currently, any caller who wants to avoid duplicate subscriptions would need to use some kind of workaround, such as:

• Avoid calling subscribe/3 more than once by tracking subscriptions in their own process state, or by checking Registry.lookup/2 directly (relying on an implementation detail of PubSub)
• Call unsubscribe/2 each time before calling subscribe/3