Disable referential integrity improvements for postgresql adapter

Hey all,

While inserting fixtures to the PostgreSQL database we are disabling/enabling triggers (referential integrity) for ALL tables that have found the following schema search path. I suggest disabling/enabling triggers only for fixtures-specific tables. For example, I have overall 20 tables but only 2 fixtures. Looks like disabling triggers only for these 2 fixture-specific tables is enough. What do you think? Maybe there are some reasons why the current approach is good?

I’m going to make a PR with these base changes https://github.com/gigorok/rails/commit/faac2bb967e29edb7acc58149bc9f58ce8e7fc98