The original fix for ReactiveX#525 breaks sequences where some observables emit
item faster than other ones. By completing too soon, the remaining
observables cannot catchup later. A very simple case is in ReactiveX#578 where
the two observables to zip emit their items sequentially.

We can fix both issues by completing whenever an observable completes
and there is no queued item. Otherwise we let the remaining observables
a chance to emit new items before completion.

The original fix for #525 breaks sequences where some observables emit
items faster than other ones. By completing too soon, the remaining
observables cannot catch up later. A very simple case is in #578 where
the two observables to zip emit their items sequentially.

We can fix both issues by completing whenever an observable completes
and there is no queued item. Otherwise, we let the remaining observables
a chance to emit new items before completion.