Allowed lateness for coGroup/join operators

[Евгений Юшин]

Hi folks

I'm surprised coGroup and join doesn't support allowedLateness feature
(while underlying WindowedStream does) in the current release (1.5/1.6).
I've opened a jira ticket with more details and ready to make appropriate
changes in API:
https://issues.apache.org/jira/browse/FLINK-10050

I'm ready to proceed with Pull Request but want to hear from you first.

Regards,
Eugen