Make postgres foreign keys deferrable.

Having deferable keys will be important for fixtures. If a constraint
is not deferable, it cannot be turned off temporarily. Since the current
schema tools are generally for creating fixture schema we'll assume
people want that feature as it is pretty handy.
Add disable/enable constraints for postgres.

Now that fixture constraints can be deferred, we can use the simple
version of disabling constraints.
Add disable/enable constraints for SQLServer.

I have no verified this, but it looks like it will work.