Remove the reference to the AIS as unused in Join
Update ais.ForeignKey to use ais.Join. Purely an internal refactoring,
to be used for the FK processing update later.
merge tjoneslo: Foreignkey use join

Merge pull request #1193 from tjoneslo/sql-layer/foreignkey-use-join
https://github.com/FoundationDB/sql-layer/pull/1193