Refactor referential_integrity.rb (#disable_referential_integrity)

[BuonOmo]

As cockroachdb/cockroach#19444 is stale, we should put a bit more care in this method that might exist for a long time.

Currently:

• It is super slow
• It is made to work out of transactions, could work better within transactions (we might need an option)
• Some indexes are not properly deleted

[BuonOmo]

Recently improved in #358. It still takes roughly half of the test time just to add and delete the foreign keys.