Wrapping update queries with joins in a subquery

[jwoertink]

Fixes #827

This changes the syntax of the UPDATE sql so when you have a join, it moves the whole WHERE clause to a subquery.

Previously, if you tried to bulk update with a join, it would throw a postgres syntax error

UPDATE ... SET x = 1 INNER JOIN.....

Now this moves it to a subquery with

UPDATE ... SET x = 1 WHERE EXISTS (.... INNER JOIN ....);