Revert "Revert "Reduce allocations when running AR callbacks.""

This reverts commit bdc1d329.

Before:
Calculating -------------------------------------
                        22.000  i/100ms
-------------------------------------------------
                        229.700  (± 0.4%) i/s -      1.166k
Total Allocated Object: 9939

After:
Calculating -------------------------------------
                        24.000  i/100ms
-------------------------------------------------
                        246.443  (± 0.8%) i/s -      1.248k
Total Allocated Object: 7939

```
begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  # gem 'rails', github: 'rails/rails', ref: 'bdc1d329'
  gem 'rails', github: 'rails/rails', ref: 'd2876141'
  gem 'arel', github: 'rails/arel'
  gem 'sqlite3'
  gem 'benchmark-ips'
end

require 'active_record'
require 'benchmark/ips'

ActiveRecord::Base.establish_connection('sqlite3::memory:')

ActiveRecord::Migration.verbose = false

ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.string :name, :email
    t.boolean :admin
    t.timestamps null: false
  end
end

class User < ActiveRecord::Base
  default_scope { where(admin: true) }
end

admin = true

1000.times do
  attributes = {
    name: "Lorem ipsum dolor sit amet, consectetur adipiscing elit.",
    email: "foobar@email.com",
    admin: admin
  }

  User.create!(attributes)

  admin = !admin
end

GC.disable

Benchmark.ips(5, 3) do |x|
  x.report { User.all.to_a }
end

key =
  if RUBY_VERSION < '2.2'
    :total_allocated_object
  else
    :total_allocated_objects
  end

before = GC.stat[key]
User.all.to_a
after = GC.stat[key]
puts "Total Allocated Object: #{after - before}"
```

[ci skip] add note for individual stub creation

Rack [already implements `redirect?` on the response object](https://github.com/rack/rack/blob/1569a985e17d9caaf94d0e97d95ef642c4ab14ba/lib/rack/response.rb#L141) so we don't need to implement our own.

It is already on Active Support

Replace `ActiveSupport::Concurrency::Latch` with `Concurrent::CountDownLatch` from concurrent-ruby.

Change AC::TestResponse to AD::TestResponse

We want to treat the response object as if it's a real response object
(not a test object), so we should only call methods that are on the
superclass.

ActionController::TestResponse was removed in d9fe10cb and caused a test
failure on Action View as its test case still refers to it.

make test runner work correctly inside engine

We shouldn't depend on specific methods imlemented in the TestResponse
subclass because the response could actually be a real response object.

In the future, we should either push the aliased predicate methods in
TestResponse up to the real response object, or remove them

[ci skip] docs: making clear that perform_caching has a limited impact

This will save Travis some precious resource since it will not need to
run 18 jobs for these Ruby implementations on every push or pull
request.

We do care about these implementations but we should fix the build
locally before having it running on travis. We would love to have
someone working on this but right now it is not our reality.

The concurrent-ruby gem is a toolset containing many concurrency
utilities. Many of these utilities include runtime-specific
optimizations when possible. Rather than clutter the Rails codebase with
concurrency utilities separate from the core task, such tools can be
superseded by similar tools in the more specialized gem. This commit
replaces `ActiveSupport::Concurrency::Latch` with
`Concurrent::CountDownLatch`, which is functionally equivalent.

fix typo in caching guide [ci skip]

Removed usage line docs [ci skip]

Removes unnecessary comments from i18n validations tests [ci skip]

These comments do not add a lot to the readability, grepability or
overall understanding of the tests, therefore I believe they can be
safely removed.

fix class name typo [ci skip]

[ci skip] change sentence about reporting bugs in contributing guide

Improve duplicable documentation [ci skip]

Remove the reference to mocha in activemodel

Activemodel is no longer dependent on mocha, so we can make the comments
more generic.

constant loading should be thread safe now, so lets remove this