Improve the support for finite streaming jobs with async operations.

Finite streaming jobs will terminate even in the presence of asynchronous in flight operations.

Looking at the AsyncWait operator, it seems that it can be mitigated by using the following interface:

https://github.com/apache/flink/blob/master/flink-streaming-java/src/main/java/org/apache/flink/streaming/api/operators/BoundedOneInput.java#L27

Mailing list thread:

http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/Stateful-Functions-Help-for-calling-remote-stateful-function-written-in-Python-td43283.html