fix xs.merge completion

[Hypnosphi]

This is intended to fix #74

The problem it that case is that MergeProducer#_stop method never gets called because of cancelled async stoppage. So when original stream gets executed second time and then completed, the ac counter goes to -1 and that doesn't lead to merged stream completion.

The idea of PR is that we can be sure about (virtual) count of active listeners right before we iterate through insArr and call _adds, because each of the incoming streams either runs already or gets started by _add.

There's still a potential problem in case if some incoming stream gets restarted by another listener. To fix that, merge should be rewritten in the manner of combine to track which of the streams actually completes.

[staltz]

Thanks a lot @Hypnosphi , this was perfect fix.