On CI we've seen a few test failures when the DatabaseSelectorTest runs
before the ConnectionHandlersMultiDbTest. This is because it's creating
2 handlers but not properly tearing them down.

Example failure:

```
Failure:
ActiveRecord::ConnectionAdapters::ConnectionHandlersMultiDbTest#test_connects_to_with_single_configuration
[/rails/activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb:241]:
Expected: 1
  Actual: 2
```

Clarify collection proxy docs

This reverts commit b46601b5, reversing
changes made to 4e6737f1.

Correct JavaScript guide example

Avoid extra package install

SQLite3: Implement `add_foreign_key` and `remove_foreign_key`

This reverts commit 141f50ef, reversing
changes made to 317efa5c.

Reason: `add_foreign_key` and `remove_foreign_key` are implemented at
#35212.

[ci skip]

I implemented Foreign key create in `create_table` for SQLite3 at
#24743. This follows #24743 to implement `add_foreign_key` and
`remove_foreign_key`.
Unfortunately SQLite3 has one limitation that
`PRAGMA foreign_key_list(table-name)` doesn't have constraint name.
So we couldn't implement find/remove foreign key by name for now.

Fixes #35207.
Closes #31343.

Some tests are running yarn install during the test.
The directory used for isolation test is not subject to yarn workspace,
and it occurs because the required package is not installed.
In order to avoid this, I fixed all necessary packages to be installed
before run test and use symlink to `node_modules`.

This is a bit complicated, as `yarn install` needs to be run in a specific
directory before running the test.
However, running `yarn install` every time run the test is expensive
when testing locally and should be avoided.

This reverts commit 38f9e41f, reversing
changes made to 5e493c3b.

Reason: The ajv@6.9.1 was released that fixes issue.

Swap `#users` jQuery selector to correct position and prevent the escaping of HTML.

Avoid ajv@6.9.0 for now

Because ajv@6.9.0 does not work with webpack.
Ref: https://github.com/epoberezkin/ajv/issues/941

https://travis-ci.org/rails/rails/jobs/491045821#L1528-L1531

Do not allow to add column without column name

Fix memory leaks in development

It makes to ease to handle all short-hand methods (e.g. validates
arguments etc).

This commit exposes all system wide view paths so that we can clear
their caches.

This commit splits the digest cache from the "details identity" cache.
Now both caches can be managed independently.

`object.transform_values!` returns `object` itself.

The docs for `ActiveRecord::Associations::CollectionProxy` describe
`ActiveRecord::Associations::Association`. I've moved them to
association and rewrote collection proxy's docs to be more applicable to
what the class actually does.`

[ci skip]

`Association#target=` invokes `loaded!`, so we no longer need to call
the `loaded!` explicitly.

Since Preloader is private API, we don't guarantee that it behaves like
older version as long as using Preloader directly. But this refactoring
fortunately also fix the Preloader compatibility issue #35195.

Closes #35195.

Fix elapsed time calculations

Fix `relation.exists?` with giving both `distinct` and `offset`

Bugfix has_many association #size when ids reader is cached and assoc…

I've found a few places in Rails code base where I think it makes sense
to calculate elapsed time more precisely by using
`Concurrent.monotonic_time`:

- Fix calculation of elapsed time in `ActiveSupport::Cache::MemoryStore#prune`
- Fix calculation of elapsed time in
  `ActiveRecord::ConnectionAdapters::ConnectionPool::Queue#wait_poll`
- Fix calculation of elapsed time in
  `ActiveRecord::ConnectionAdapters::ConnectionPool#attempt_to_checkout_all_existing_connections`
- Fix calculation of elapsed time in `ActiveRecord::ConnectionAdapters::Mysql2Adapter#explain`

See
https://docs.ruby-lang.org/en/2.5.0/Process.html#method-c-clock_gettime
https://blog.dnsimple.com/2018/03/elapsed-time-with-ruby-the-right-way

Related to 7c454214

The `distinct` affects (reduces) rows of the result, so it is important
part when both `distinct` and `offset` are given.

Replacing SELECT clause to `1 AS one` and removing `distinct` and
`order` is just optimization for the `exists?`, we should not apply the
optimization for that case.

Fixes #35191.

Relation is not best place to do this.

Address test_belongs_to_does_not_use_order_by failure