Support auto-repartition when joining on subcomponent of composite key

[big-andy-coates]

This came up in a discussion with a customer:

Given a stream with a composite key, e.g. Struct<X INT, Y INT>, KSQL should allow a join on the sub components of the key:

SELECT * from A join B on A.ROWKEY.x = B.ROWKEY.y;

This discussion was outside the context of structured key support, but it seems it would need this support first.

[miguno]

See #824 for structured key support.

[mjsax]

Given the work on structure keys, this should be supported now. Closing this ticket.