KeyedJoins that depend on parent context from onStart crash when accessing remote parts

If a keyed join helper depends on info from the parent context that it retrieves during onStart(), then currently KJ will cause a crash when accessing remote parts.
The remote KJ service is not calling onStart, nor does it have a serialized parent context.

Gavin pointed out that could be fixed by serializing the parent context, but would be better if it serialized the keyed filter.