Add option to stop swallowing errors on callbacks.

Currently, Active Record will rescue any errors raised within
after_rollback/after_create callbacks and print them to the
logs. Next versions of rails will not rescue those errors anymore,
and just bubble them up, as the other callbacks.

This adds a opt-in flag to enable that behaviour, of not rescuing
the errors.
    Example:
      # For not swallow errors in after_commit/after_rollback
      config.active_record.errors_in_transactional_callbacks = true

[fixes #13460]
Rack is released too
Merge pull request #16537 from arthurnn/stop_swallowing_errors_2

Add option to stop swallowing errors on callbacks.
fix broken link [ci skip]
[ActiveJob] Add queue_name_prefix so all queue are prefixed .
We can split queues on a per environement basis as well as seggregate per application so we can split queues on a per env basis

ActiveJob::Base.queue_name_prefix = 'foo'
Merge pull request #16546 from aditya-kapoor/remove-broken-link

fix broken link to rails blog [ci skip]
Add activejob guide to documents.yml [ci skip]
Merge pull request #16539 from seuros/activejob

[ActiveJob] Add queue_name_prefix
Merge pull request #16547 from cristianbica/add-active-job-guide-links

Add activejob guide to documents.yml [ci skip]
Fixes the `Relation#exists?` to work with polymorphic associations.

Fixes #15821.
Fix broken link to Upgrading Ruby on Rails Guide

in the 4.2 Release Notes [skip ci]

Change to link suggested by @zzak [skip ci]
Merge pull request #16545 from jonatack/patch-9

Fix broken link to Upgrading Ruby on Rails Guide [ci skip]
Fix after_commit warning message
Merge pull request #16548 from arthurnn/fix_warn_message

Fix after_commit warning message
Deprecate TagAssertion instead of removing
Removed warning
Point to unreleased deprecated_sanitizer
Remove to_s from reflection.type in add_constraints

The instance var is already saved as a string in the initialization
method of AssociationReflection.

See https://github.com/rails/rails/blob/master/activerecord/lib/active_record/reflection.rb#L273
Merge pull request #16550 from eileencodes/remove-unnecessary-to_s-from-add_constraints-method

Remove to_s from reflection.type in add_constraints
Merge pull request #16542 from seamusabshere/numerify-pool-checkout-timeout-from-urls-4-1-stable

Make sure :checkout_timeout and :dead_connection_timeout are numbers
Conflicts:
	activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb