Don't disable referential integrity for the same table twice

[buehmann]

The following caught my eye when running connection.disable_referential_integrity { ... } in a Rails console (using v4.2.18 of the adapter):

...
  SQL (99.4ms)  ALTER TABLE [dbo].[accounts] CHECK CONSTRAINT ALL
  SQL (80.5ms)  ALTER TABLE [dbo].[accounts] CHECK CONSTRAINT ALL
  SQL (85.7ms)  ALTER TABLE [dbo].[accounts] CHECK CONSTRAINT ALL
  SQL (107.9ms)  ALTER TABLE [dbo].[debt_claim_postings] CHECK CONSTRAINT ALL
  SQL (83.6ms)  ALTER TABLE [dbo].[debt_claim_postings] CHECK CONSTRAINT ALL
  SQL (86.2ms)  ALTER TABLE [dbo].[debt_claim_postings] CHECK CONSTRAINT ALL
  SQL (87.2ms)  ALTER TABLE [dbo].[debt_claim_postings] CHECK CONSTRAINT ALL
  SQL (88.5ms)  ALTER TABLE [dbo].[debt_claim_postings] CHECK CONSTRAINT ALL
  SQL (82.9ms)  ALTER TABLE [dbo].[debt_claim_postings] CHECK CONSTRAINT ALL
  SQL (84.8ms)  ALTER TABLE [dbo].[tx_pins] CHECK CONSTRAINT ALL
...

Apparently duplicates are not eliminated when determining the set of tables with foreign key constraints.

In our case the adapter emitted 2 * 256 ALTER TABLE ... (NO)CHECK CONSTRAINT ALL statements instead of the sufficient 2 * 167.

It would be lovely to see this fix in the 4.2 line as well.

[thomasfedb]

@metaskills this PR seems reasonable. Could it be merged?

[metaskills]

Your call @thomasfedb but yes.

[wpolicarpo]

Thanks for your contribution @buehmann.