PostgreSQL, warn once per connection per missing OID. Closes #14275.

[Yves Senn & Matthew Draper]

(cherry picked from commit 9f62344d9552f0a58e4aa43ca6f1a7fec273a04e)

This is a partial backport of #14692.

Conflicts:
    activerecord/CHANGELOG.md
    activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
    activerecord/test/cases/adapters/postgresql/postgresql_adapter_test.rb

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/lib/active_record/connection_adapters/postgresql/database_statements.rb
	activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
	activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb