Side Input callbacks should fire after a window has expired in addition to when the trigger would fire

Otherwise a streaming pipeline where elements trying to access a window that has expired will not progress, even though we can be certain that there will never be inputs in that window.

Imported from Jira [BEAM-3529](https://issues.apache.org/jira/browse/BEAM-3529). Original Jira may contain additional context.
Reported by: tgroh.

Start by reading the issue and the linked BEAM-3529 Jira ticket for the missing context, then trace the side input callback and window-expiration handling in the Beam codebase. Done means callbacks occur after a window expires, as well as when the trigger fires, so streaming pipelines can progress without later input for that window.

Written by the indexing model from the issue text.