Using `each_with_object` instead of `reduce`

This way no new object allocation is taking place. Thanks @jeremy for
the suggestion!
add I18n support for `:placeholder` HTML option is passed to form fields
just use the placeholder tag value if it is passed as a String
Merge pull request #16560 from robin850/rbx-duration-eql

Define the Duration#instance_of? method
Merge pull request #16626 from adomokos/replacing-each-with-reduce

Replacing an each with reduce
Implemented enqueue_at for ActiveJob's Backburner adapter
Active Job guide: Add info about queue_name prefix and clarity supporter features
Roll back open transactions when the running thread is killed.
Merge pull request #16622 from matthewd/default-debug

Don't fight against ourselves for the default log level
Merge pull request #16642 from cristianbica/aj-doc-queue-name-prefix

Active Job Guide: Example of config queue_name_prefix [ci skip]
Merge pull request #16641 from cristianbica/aj-backburner-enqueue-later

Implemented enqueue_at for ActiveJob's Backburner adapter
Don't calculate in-place changes twice

Now that `changed_attributes` includes in place changes, we don't need
to override these methods in Active Record. Partially fixes the
performance regression caused by #16189
Merge pull request #16645 from sgrif/sg-perf-regression

Don't calculate in-place changes twice
[ActionMailer] require activejob railtie
Merge pull request #15932 from joseluistorres/issue-11957-uuids

adding the exception RecordNotFound to UUID find #11957
Merge pull request #16598 from seuros/master

[ActionMailer] require activejob railtie
Cache the value of `changed_attributes` when calling `changes_applied`

`changes_applied` calles `changes`, which will call `changed_attributes`
multiple times in a loop. This method actually performs work now, so we
should cache the results while looping over it when we know it cannot
change.
Use system /tmp when testing actionpack

https://github.com/rails/rails/commit/c64bff2c87ebf363703c63ecd4a96d56a1a78364
added support and enabled parallel execution of the actionpack tests.

However it introduced https://github.com/rails/rails/commit/c64bff2c87ebf363703c63ecd4a96d56a1a78364
since one cannot connect to a socket file that's inside a Vagrant synced folder
due to security restrictions, and DRb tries to.

Also rename the temporary files to make it obvious that they're rails-related,
since now they're placed outside the project's directory.

Fixes https://github.com/rails/rails/commit/c64bff2c87ebf363703c63ecd4a96d56a1a78364
Clarify that unknown keys will become HTML attrs

Docs for all the other form tag helpers in this
file already clarify this.

[ci skip]
Remove erroneous form_tag option docs

This is true of the first param (url_for_options),
not of the second options param which is being
documented here.

[ci skip]