Add 'garbage collection' hold when receive late element

We currently add a 'garbage collection' hold in WatermarkHold (invoked via ReduceFnRunner) if the closing behavior is FIRE_ALWAYS. This means an element which has come in too late for a data holds and an end-of-window hold may end up setting no hold at all. As a result, the eventual pane containing that element may end up dropped as being too late.

Imported from Jira [BEAM-311](https://issues.apache.org/jira/browse/BEAM-311). Original Jira may contain additional context.
Reported by: mshields822.

Start by tracing the interaction between WatermarkHold and ReduceFnRunner when closing behavior is FIRE_ALWAYS, focusing on late elements and existing hold selection. Done means a late element retains a garbage-collection hold alongside the relevant end-of-window behavior so its eventual pane is not dropped as too late; add or update the corresponding tests found during that trace.

Written by the indexing model from the issue text.