Add setter and deprecation for configurations hashes

In chat Sam Saffron asked how to use the setter now that configurations
is no longer a hash and you need to do AR::Base.configurations["test"]=.

Technically you can do `ActiveRecord::Base.configurations = { the hash
}` but I realized the old way throws an error and is unintuitive.

To aid in the transition from hashes to objects this PR makes a few
changes:

1) Re-adds a deprecated hash setter `[]=` that will add a new hash
to the configurations list OR replace an existing hash if that
environment is already present. This won't be supported in future Rails
versions but a good error is important.

2) Changed to throw deprecation warnings on the methods we decided to support
for hash conversion and raise on the methods we don't support.

3) Refactored the setter/getter hash deprecation warnings messages and
rewrote them.

Getters message:

```
DEPRECATION WARNING: `ActiveRecord::Base.configurations` no longer
returns a hash. Methods that act on the hash like `values` are
deprecated and will be removed in Rails 6.1. Use the `configs_for`
method to collect and iterate over the database configurations.
```

Setter message:

```
DEPRECATION WARNING: Setting `ActiveRecord::Base.configurations` with
`[]=` is deprecated. Use `ActiveRecord::Base.configurations=` directly
to set the configurations instead.
```

4) Rewrote the legacy configurations test file to test all the public
methods in the DatabaseConfigurations class.

There is too much to say about it for a CHANGELOG entry, and linking to
the original PR could mislead if there are later changes as already
happened with the gem dependency, so just a one-liner.

For final we'll up to date documentation.

Let Zeitwerk be a dependency of Active Support

Use Dir#children and Dir#each_child in ActiveSupport::Cache::FileStore

Both methods were introduced in Ruby 2.5 and fit this use case very
nicely: listing a directory's entries excluding the "." and ".." nodes.

The private method #exclude_from was removed as it no longer serves
its original purpose.

Adding enque time tracking and logging

Motivation:
  - Currently we have 2 seperate monkey patches in place for tracking
  enqueded time for 2 seperate workers. It seems that activejob could be
  a source of truth for how long an item has been enqued so that we can
  easily use it for consistent monitoring across workers/apps to ensure
  that jobs are running at an acceptable speed.

Changes:
  - Add an enqueded at attribute and serilization tooling.
  - Add a method to get how long a job has been enqueded for.
  - Add a logging item to show how long a job was enqued prior to the
  perform method firing.

Revert "Chaining named scope is no longer leaking to class level querying methods"

Zeitwerk is a strong dependency, planned to replace AS::Dependencies. A
line in the generated Gemfile does not convey this as much.

This reverts #32380, since this may cause that silently leaking
information when people upgrade the app.

We need deprecation first before making this.

Cached collections only work if there is one template

Cached collections only work if there is one template.  If there are
more than one templates, the caching mechanism doesn't have a key.

I believe the current style does not clearly communicate that we are
ignoring non-existing autoload paths altogether. Your eyes may even be
looking for an else clause that is easy to miss but that does not exist.

With the early `next` and code comment the loop reads better for my
taste.

Address to useless assignment `formats = nil` after #35254

https://codeclimate.com/github/rails/rails/pull/35254

Clean for fix done in #35229. Removed unused arguments for `build_lookup_context `

Convert lookup context's to a stack for fixing #35222 and #34138

Fix database configurations building when DATABASE_URL present

Since production applications typically run with log level info and
email adresses should be considered as sensitive data we want to prevent 
them from ending up in the logs. In development mode (with log level
debug) they are still logged as part of the Mail::Message object.

This reverts commit eec3e28a, reversing
changes made to 5588fb48.

Reason: Marking as loaded without actual loading is too greedy optimization.

See more context #35239.

Closes #35239.

[Edouard CHIN & Ryuta Kamizono]

Add Worker Pool section to Action Cable configuration docs

Action Cable: allow multiple instances of Server::Base with different configs

That allows us to create a separate, isolated Action Cable server
instance within the same app.

Fixed the CI failure https://travis-ci.org/rails/rails/jobs/492291248#L1185-L1191.

Fix DatabaseSelector test that fails sometimes

Currently custom attributes are always qualified by the table name in
the generated SQL wrongly even if the table doesn't have the named
column, it would cause an invalid SQL error.

Custom attributes should only be qualified if the table has the same
named column.

Zeitwerk integration

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
```

Support testing of non-ActionDispatch-routed apps.

If generate application without specified options,`actioncable` and
`activestorage` loads by default.

This commit keeps a stack of lookup contexts on the ActionView::Base
instance.  If a format is passed to render, we instantiate a new lookup
context and push it on the stack, that way any child calls to "render"
will use the same format information as the parent.  This also isolates
"sibling" calls to render (multiple calls to render in the same
template).

Fixes #35222 #34138