Rails 6: Updates for bulk insert/upsert

[aidanharan]

Bulk inserts akin to the bulk updates provided by update_all and bulk deletes by delete_all was added to Rails in rails/rails#35077.

Other adapters support skipping or upserting duplicates through the ON CONFLICT syntax for Postgres (9.5+) and Sqlite (3.24+) and ON DUPLICATE KEY UPDATE syntax for MySQL.

SQL Server, however, does not support skipping or upserting duplicates. There are some workarounds in https://michaeljswart.com/2017/07/sql-server-upsert-patterns-and-antipatterns/ but they will require more work to implement and test.

This PR implements the functionality to support insert returning. This will run the previous skipped tests such as https://github.com/rails/rails/blob/master/activerecord/test/cases/insert_all_test.rb#L75

I have coerced the QueryCacheExpiryTest#test_insert_all test as it should have been checking whether the adapter supported the supports_insert_on_duplicate_skip?/supports_insert_on_duplicate_update? functionality before calling insert/upsert.

References:

• Add insert_many to ActiveRecord models rails/rails#35077
• https://michaeljswart.com/2017/07/sql-server-upsert-patterns-and-antipatterns/

[aidanharan]

These are already set to false by the abstract adapter but think it better to duplicate the methods here so that it's clear what the MSSQL adapter supports.