DirectRunner does not update reference to currentRestriction when running in SDF

I have an SDF implementation that looks like so:

 
```

class MyRestrictionTracker {
MyRestriction restriction;

currentRestriction() { return restriction;
}

tryClaim(MyPosition position) {
this.restriction = new MyRestriction(position)
}
}
```

I ran this on the DirectRunner, and the restriction would never advance: It would get stuck on the very first value.

I also ran this on DataflowRunner, and the problem did not exist there: This ran fine.

I was able to fix this on the DirectRunner (it works well on Dataflow as well) by changing the restriction to be mutable. Something like this:

 
```

class MyRestrictionTracker {
MyRestriction restriction;

currentRestriction() { return restriction;
}

tryClaim(MyPosition position) {
this.restriction.position = position;
}
}
```

This looks like an execution issue with SDF on DirectRunner: The DirectRunner is likely storing a reference to `currentRestriction()` and never updating it as it runs.

I'm happy to fix this on the DirectRunner - I would just like to find pointers : )

Imported from Jira [BEAM-14387](https://issues.apache.org/jira/browse/BEAM-14387). Original Jira may contain additional context.
Reported by: pabloem.