retry() hangs when it is followed by a group by with >= 1024 items

[neerajrj]

For an operator chain like o.retry().groupBy().... if the source Observable has >= 1024 items the JVM seems to hang. Repeated unsubscribe calls seem to be sent by retry() upwards
https://gist.github.com/neerajrj/8bb9106ca1b81e08a40f

[benjchristensen]

Fixed in #1901