sql: support ActiveRecord through postgresql adapter

[cuongdo]

These are the features needed to support ActiveRecord using its postgresql database adapter:

• sql: support unnest() built-in function #12785 support unnest built-in
• sql: extract DTable-typed render expressions as cross-join #11215 extract set-returning functions from renders to cross-joins
• sql: support col_description() built-in #12783 support col_description built-in
• sql: unwrap inner quotes in OID typecasts #12153: unwrap inner quotes in OID typecasts
• sql: support format_type builtin function #12151: support format_type built-in
• sql: current_schema{,s}() builtins should return public, not database name #12148: current_schema{,s}() builtins should return public, not database name
• sql: support array_in builtin function #12139: support array_in built-in
• sql: Implement pg_catalog.pg_collation as virtual table #12976: pg_catalog.pg_collation
• sql: ActiveRecord expects precise postgres type names  #12145: ActiveRecord expects precise type names
• sql: support pg_try_advisory_{lock,unlock}(int) builtin #13486: try_advisory_lock() (Rails 5)
• sql: support current_database builtin #13485: current_database() (Rails 5)
• sql: improve regproc and other regfoo OID types  #13567: Improved support for regfoo types
• sql: support rolling back nested transactions containing schema changes #10735: SAVEPOINT

[cuongdo]

cc @jordanlewis @nvanbenschoten

[jordanlewis]

After #13429 and a workaround for #13468, we manage to successfully make it past all of ActiveRecord's type management and schema creation routines - rake db:migrate on a fairly complex example works without a hitch. Next step is working through and validating some actual CRUD operations via the ORM.

[jordanlewis]

So it turns out common table expressions aren't used before Rails/ActiveRecord 5.0, which was released in June 2016. We could probably claim Rails/ActiveRecord 4.2.6 support without too much more work - notably we wouldn't need #7029 or #13156.

[jordanlewis]

In addition, in order to support Rails 5.0+ without supporting correlated subqueries, we'll need to wait until rails/rails#27743 is released in Rails 5.0.2.

[jordanlewis]

Once rails/rails#27949 makes it into a release, we won't need common table expressions at all.

[jordanlewis]

Rails uses savepoints during transactions. Looks like we'll need to support them. Added #10735 to the list.

[jordanlewis]

We changed strategies and now provide an adapter for CockroachDB support in ActiveRecord. See https://github.com/cockroachdb/activerecord-cockroachdb-adapter and the docs.