reactivity and better support to _allSubscriptionsReady

### Scenario

*I want to have in the client side of my app a loading message while the data loading (subscriptions) is still not completed (ready)*.

In my specific case, my app does some requests to third party services inside `publish` methods and the user is supposed to wait for some time before the publish is `ready`. So, a loading message is necessary.

Would be nice to have control over when **all** your data loading is completed or not, so you could decide if you wanna wait more or show what you already have based on an incomplete database.

### Problem

The function `Meteor._LivedataConnection._allSubscriptionsReady` tells us when all subscriptions are ready, but it has some issues:

* it is documented as a simple *"Hack for spiderable package"*. I do not feel comfortable relying my code on this.

* It is not reactive. No one wants to be checking it all the time.

### Solution

* Promote `_allSubscriptionsReady` from an obscure function to something explicitly supported.

* Make it reactive.

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This pull request solves only the reactivity question, just adding a line of code. I haven't solved the other issue because it is purely a design and organizational decision.