Recording breaking changes to Streaming API

[Stephan Ewen]

Hi all!

Some committers have started an effort to consolidate the streaming API for
the next release.

This should be the last major change, before graduating the API, moving it
out of "flink-staging" and regarding it as stable.

Because we have people that seriously use that API already, we need to
record all breaking changes (API and behavior) and attach that to the next
release notes.

I suggest that we record all breaking changes to the streaming API here:

https://docs.google.com/document/d/1itrALuaj_mmBMFqfEI1pOt8m8R1BIsmVFS_uuabsJIE

I have given most committers edit permissions. Ping me if you want edit
permissions.

Greetings,
Stephan