Pro: subscriptions can't be atomic

[rmosolgo]

The current Pusher and Ably implementations have the same flow:

1. HTTP Post subscription { ... } which returns a X-Subscription-ID header
2. Client subscribes to the vendor's channel with that name
3. Updates come over the channel, until the client unsubscribes from it

However, some server events may occur between the response in 1. and the subscribe in 2., and the framework doesn't have any way to account for this.

Somehow, "atomic" subscriptions should be possible with GraphQL-Pro. (I don't think it's possible with ActionCable -- if clients disconnect, then events are lost. But at least Ably will replay events for clients.)

[rmosolgo]

Discussing this in more detail here: #5138