Allow "sync" verification about pubsubhubbub

Because it can't work if we want the feed publisher's approval.
If you're a subscriber and want 'sync', but the feed is protected, you won't get an verification right away, as the hub needs to ask the publisher.

In my experience, having 2 different modes made things a lot messier for a lot of people.
I'm in favor of using the mode that allows everyone to get stuff done, rather than 2 modes where people pick the wrong one and eventually figure out that the one they picked didn't work.

Dammit! Wrong Github account... but the previous comment was from me!

[evanp]

Seems like a negotiation process at subscription time, then.

if (topic requires async) {
    if (subscriber only does sync) {
        hub returns 400 failure;
    } else {
        do async;
  }
} else {
    if (subscriber only does sync) {
        do sync;
    } else if (subscriber only does async) {
        do async;
    } else {
        do either!;
    }
}

Complexity... and weirdness. Why would we let subscribers assume they can do "sync" when they can't?

Another problem with the sycn approach is that it the subscriber cannot know before actually subscribing whether the feed requires a publisher validation... and if it does, then, there is no way this can be sync'ed, because the publisher may take time (at least user authorization) before it validates the subscription...