returns true

Not only postgresql or mysql2 adapter, Oracle enhanced adapter
whose default isolation level is read commited, passes these two test cases.

`ConcurrentTransactionTest#test_transaction_per_thread`
`ConcurrentTransactionTest#test_transaction_isolation__read_committed`

```ruby
$ ARCONN=oracle bin/test test/cases/transactions_test.rb:961 -v
Using oracle
Run options: -v --seed 18865

ConcurrentTransactionTest#test_transaction_per_thread = 0.98 s = .

Finished in 1.061036s, 0.9425 runs/s, 5.6549 assertions/s.

1 runs, 6 assertions, 0 failures, 0 errors, 0 skips
```

```ruby
$ ARCONN=oracle bin/test test/cases/transactions_test.rb:979 -v
Using oracle
Run options: -v --seed 13341

ConcurrentTransactionTest#test_transaction_isolation__read_committed = 1.85 s = .

Finished in 1.928637s, 0.5185 runs/s, 10.3700 assertions/s.

1 runs, 20 assertions, 0 failures, 0 errors, 0 skips
$
```

Also, regardless it is a file based or memory based these tests could fail with SQLite3Adapter.
(Extra CR added to make lines shorter)

```ruby
$ ARCONN=sqlite3 bin/test test/cases/transactions_test.rb:961 -v
Using sqlite3
Run options: -v --seed 18815

ConcurrentTransactionTest#test_transaction_per_thread = /home/yahonda/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/sqlite3-1.3.13/lib/sqlite3/statement.rb:108:in `step':
SQLite3::BusyException: database is locked: UPDATE "topics" SET "approved" = ?, "updated_at" = ? WHERE "topics"."id" = ? (ActiveRecord::StatementInvalid)
```

```ruby
$ ARCONN=sqlite3 bin/test test/cases/transactions_test.rb:979 -v
Using sqlite3
Run options: -v --seed 25520

ConcurrentTransactionTest#test_transaction_isolation__read_committed = 0.12 s = E
/home/yahonda/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/sqlite3-1.3.13/lib/sqlite3/statement.rb:108:in `step':
SQLite3::BusyException: database is locked: UPDATE "developers" SET "salary" = ?, "updated_at" = ?, "updated_on" = ? WHERE "developers"."id" = ? (ActiveRecord::StatementInvalid)
```

`through_reflection` and `source_reflection` are used only in the class.

`associated_records_by_owner` had returned customizing result before
calling `associate_records_to_owner` for through association subclasses.
Since #22115, `associate_records_to_owner` is called in the method and
not returned owner and result pairs. Removing the method will reduce
method call and block call nesting.

Ensure that `bin/rails db:migrate` with specified `VERSION` reverts
all migrations only if `VERSION` is `0`.
Raise error if target migration doesn't exist.

```
[NoMethodError]: undefined method `state' for nil:NilClass  Method:[rescue in block in refresh]
```

In `within_new_transaction`, there is the possibility that
`begin_transaction` returns a `nil`. (i.e.: so `transaction = nil`)
So this method is checking `transaction` for nil in 2 spots.

Unfortunately, there is one line that is not checking `transaction` for `nil`
That line, `commit_transaction`, throws an exception for us in AR 5.0.0.1

The problem with the method is finally realized in the error checking itself.
it calls `transaction.state` (i.e.: nil.state) and that is the final exception
raised.

The actual underlying (user) issue is hidden by this line.

Solution is test transaction for nil.

A relation includes `klass`, so it can not be used as it is if current
class is different from `current_scope.klass`. It should be created new
relation by current class to respect the klass and STI constraint.

Fixes #17603.
Fixes #23576.

This is partially fixed by e617fb57 when through association has already
loaded. Otherwise, second level through association should respect
`preload_scope`.

Fixes #30242.
Closes #30076.

[Ryuta Kamizono & CicholGricenchos]

If through association has already loaded, `source_type` is ignored to
loaded through records. The loaded records should be filtered by
`source_type` in that case.

Fixes #30904.

If a relation has eager-loading values, `count` and `exists?` works
properly, but `update_all` and `delete_all` doesn't work due to missing
`apply_join_dependency`. It should be applied to work consistently.

Fixes #28863.

Fixes #30315.

* `HasManyThroughCantDissociateNewRecords` and `HasManyThroughCantAssociateNewRecords`
are no longer used since f6b12c11.
* `ReadOnlyAssociation` is no longer used since 0da426be.

Follow up of #31004.

This pull request addresses these 17 failures when tested with Oracle enhanced adapter.
All of these failures are due to the incorrect number of queries. Here is the first one.

```ruby
$ ARCONN=oracle bin/test test/cases/nested_attributes_test.rb:1086
Using oracle
Run options: --seed 27985

F

Finished in 0.874514s, 1.1435 runs/s, 1.1435 assertions/s.

  1) Failure:
TestHasManyAutosaveAssociationWhichItselfHasAutosaveAssociations#test_circular_references_do_not_perform_unnecessary_queries [/home/yahonda/git/rails/activerecord/test/cases/nested_attributes_test.rb:1086]:
6 instead of 3 queries were executed.
Queries:
select us.sequence_name from all_sequences us where us.sequence_owner = :owner and us.sequence_name = :sequence_name
INSERT INTO "SHIPS" ("NAME", "ID") VALUES (:a1, :a2)
select us.sequence_name from all_sequences us where us.sequence_owner = :owner and us.sequence_name = :sequence_name
INSERT INTO "SHIP_PARTS" ("NAME", "SHIP_ID", "UPDATED_AT", "ID") VALUES (:a1, :a2, :a3, :a4)
select us.sequence_name from all_sequences us where us.sequence_owner = :owner and us.sequence_name = :sequence_name
INSERT INTO "TREASURES" ("LOOTER_ID", "LOOTER_TYPE", "SHIP_ID", "ID") VALUES (:a1, :a2, :a3, :a4).
Expected: 3
  Actual: 6

1 runs, 1 assertions, 1 failures, 0 errors, 0 skips
$
```

Since https://github.com/rsim/oracle-enhanced/pull/1490 Oracle enhanced adapter drops its own schema caching called OracleEnhancedAdapter.cache_columns` to use Rails scehema cache
generated by `db:schema:cache:dump`.

By this change some extra sql statements executed at ActiveRecord unit test, which can be
fixed by adding the sql statement to `oracle_ignored`.

* All 17 failures fixed by this pull request:
```ruby
ARCONN=oracle bin/test test/cases/nested_attributes_test.rb:1086
ARCONN=oracle bin/test test/cases/locking_test.rb:308
ARCONN=oracle bin/test test/cases/locking_test.rb:365
ARCONN=oracle bin/test test/cases/dirty_test.rb:351
ARCONN=oracle bin/test test/cases/dirty_test.rb:334
ARCONN=oracle bin/test test/cases/autosave_association_test.rb:192
ARCONN=oracle bin/test test/cases/associations/has_many_associations_test.rb:950
ARCONN=oracle bin/test test/cases/associations/has_many_associations_test.rb:1059
ARCONN=oracle bin/test test/cases/autosave_association_test.rb:627
ARCONN=oracle bin/test test/cases/autosave_association_test.rb:607
ARCONN=oracle bin/test test/cases/autosave_association_test.rb:617
ARCONN=oracle bin/test test/cases/autosave_association_test.rb:641
ARCONN=oracle bin/test test/cases/associations/has_many_through_associations_test.rb:546
ARCONN=oracle bin/test test/cases/associations/has_many_through_associations_test.rb:297
ARCONN=oracle bin/test test/cases/associations/has_many_through_associations_test.rb:586
ARCONN=oracle bin/test test/cases/associations/has_many_through_associations_test.rb:172
ARCONN=oracle bin/test test/cases/associations/has_many_through_associations_test.rb:269
```

`join_scopes` in `PolymorphicReflection` is passed aliased `table`, so
it should be respected for `source_type_scope`.

Closes #13969.
Fixes #13920.
Fixes #15190.

Replace the primary key type `integer` in docs with `bigint`.

ref #26266

- It is present in Rails 5.1.3 already.
- Backport commit https://github.com/rails/rails/commit/9efa0b70a638dcfdc05b30ab530f89f2847bb9c2

```
% git grep -n trough
activerecord/test/cases/associations/has_many_through_associations_test.rb:1253:  def test_has_many_trough_with_scope_that_has_joined_same_table_with_parent_relation
```

using serializable isolation would prevent the duplicate insert as done in the example from happening

and abstract `SchemaDumper#extensions` is now an empty method.

Since #30337, every database adapter has its own `SchemaDumper`.
`extensions` are only supported by PostgreSQL database and postgresql database adapter.