ReduceFnRunner should GC based on current WM, not on a fired Timer

ReduceFnRunner currently uses the time of a timer to determine if it is the garbage collection timer. Instead, it should examine the time a window is expired and the current input watermark to determine if the window should be Garbage Collected. It should additionally ignore all timers for windows that have already been expired.

Imported from Jira [BEAM-576](https://issues.apache.org/jira/browse/BEAM-576). Original Jira may contain additional context.
Reported by: tgroh.

Start with ReduceFnRunner and trace how it identifies garbage-collection timers using the window expiration time and current input watermark. Verify the handling of timers for windows that are already expired. Done means GC decisions use the current watermark rather than a fired timer's time, and expired-window timers are ignored; no specific test or file is named.

Written by the indexing model from the issue text.