waitOn and subscriptions should be collected just like hooks

@tmeasday,

Keep in mind that waitOn now automatically uses the loading hook. So if
you want manual control over what to do with the waitlist (e.g. you
don't automatically want to use the loading hook, then use the
subscriptions option instead. Hopefully the implementation should be
clear.

see:
https://github.com/EventedMind/iron-router/issues/879