Rewrite .subscribe() to fix various bugs

[marcoscaceres]

The following are not testable/observable, but spec bugs nonetheless:

• Fix PushSubscriptionOptionsInit usage.
• Do less work in parallel.
• Handle userVisibleOnly member.
• Resolve promises using networking tasks source.

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[marcoscaceres]

Our implementation runs one of the steps out of order: the userVisibleOnly requirement is only checked quite a late in the process, as there may be (toady hypothetical) cases in which we don't require it, e.g. for extensions.

Ok, I'll move that to async part and just add a note. The ordering when that happens doesn't matter too much.