Fix race condition testing for job execution order

On most filesystems file ctime is limited to 1 second granularity, which means that on
faster computers multiple simple jobs (for instance dummy TestJob) can finish within the
same second.

The execution order test in ActiveJob integration tests relies on multiple TestJobs
writing files then comparing the ctime. As a result integration tests would sometimes
fail as the ctime of the files written by these TestJobs could have coincidental ctimes
making the comparison for job order fail.

This commit adds a far more precise execution time (to the extent that the Ruby Time
class allows) to the file created by TestJob, and updates the execution order assertion
to use it, removing the race condition.

Erroneously removed in 58910dc7.

The stubbing was a regression test to ensure `time_zone_select` wasn't implemented with
`grep`. Rename the test and add a comment to make the intent clearer.

We never touch the index, so don't bother.

When calling `test_time_zone_select_with_priority_zones_as_regexp` it would
define `=~` on the fake zones, but it would never be cleaned up because of
the zone cache.

Nuke it so `test_time_zone_select_with_priority_zones_as_regexp_using_grep_finds_no_zones`
accidentally find any zones because of `=~` being implemented.

Removed Mocha from Action View

This method needs to ensure that if a change happens, it is going to be registered.
With this refactor suggested by @matthewd race conditions do not matter because if
no file is watched, nothing is done. And as long as some invocation sets the flag
to true, it will stay true.

The refactor leaves a race condition in which two simultaneous threads that watch
some of the files passed do the actual work in `any?`, whereas the mutex guaranteed
that was done at most once. But this is considered to be a better tradeoff.

listen is calling us from its own thread, we need to synchronize reads and writes to this flag.

[ci skip] Update dirty.rb: documentation fix.

Add example for AR::Persistence#toggle

Add missing example for cookies.encrypted [ci skip]

Example of setting data attributes for image_tag

Remove arity check for `RouteSet#draw`

This code was added for migration from Rails 3.1 to upper,
now we are developing Rails 5.

add deprecations for a smooth transition after #22215

Fix test failure in `adapters/mysql/active_schema_test.rb`

Follow up to #21601.

This reverts commit 8246b593.

There was concern about this modifying the behavior of past migrations.
We're going to add an way to modify the migration generator instead.

It's often the case that you want to have an option that you cannot
specify at the database level, but want applied to *all* tables that you
create. For example, you might want to specify `ROW_FORMAT=DYNAMIC` to
not have to limit text columns to length 171 for indexing when using
utf8mb4. This allows an easy way to specify this in your database
configuration.

While this change affects both MySQL and MySQL2, the test only covers
MySQL2, as the legacy mysql adapter appears to always return ASCII
strings, and is tangential to what we're actually doing.

Change Enumerable#sum to use inject(:sym) specification

Not only does this make for simpler, more obvious code, it's also more performant:

    require 'benchmark/ips'

    module Enumerable
      def old_sum(identity = 0, &block)
        if block_given?
          map(&block).old_sum(identity)
        else
          inject { |sum, element| sum + element } || identity
        end
      end

      def new_sum(identity = 0, &block)
        if block_given?
          map(&block).new_sum(identity)
        else
          inject(:+) || identity
        end
      end
    end

    summable = (1..100).to_a # sum is 5050

    Benchmark.ips do |x|
      x.report("old_sum") { summable.old_sum }
      x.report("new_sum") { summable.new_sum }
      x.compare!
    end

    # Calculating -------------------------------------
    #              old_sum    10.674k i/100ms
    #              new_sum    14.542k i/100ms
    # -------------------------------------------------
    #              old_sum    117.350k (± 7.1%) i/s -    587.070k
    #              new_sum    154.712k (± 3.8%) i/s -    785.268k
    #
    # Comparison:
    #              new_sum:   154712.1 i/s
    #              old_sum:   117350.0 i/s - 1.32x slower

More benchmarks [here](https://gist.github.com/tjschuck/b3fe4e8c812712376648), including summing strings and passing blocks.  The performance gains are less for those, but this version still always wins.

Change configuration-guide example [ci skip]

Fix a typo [ci skip]

Use an example from a default Rails app (4.2.5) rather than an outdated one in the Configuring Rails Components section.
I picked config.time_zone as it currently is the only 'setting for Rails' left in a default config/application.rb.

I stumbled upon this with investigating autoloading in a legacy app, which still included the example comment "# config.autoload_paths += %W(#{config.root}/extras)". Usually adding app/* directories to autoload_paths isn't necessary, so also finding this example within the current docs was a bit confusing initially.

[ci skip]

Remove newlines from start of logs

[ci skip]

Since the "Getting Started" guide no longer uses the scaffold generator
we should rewrite references to that in the testing guide.

The functional testing section was quite heavily based on such a
scaffold test. I changed it to use `generate scaffold_controller`
instead so that we can build up on the model foundation we already have.

[ci skip] Indentation in API app guide 

Fixes #22311

ActiveModel::Dirty module documentation fix.

Rename 'key' to 'lock_id' or 'lock_name' for advisory locking

This reverts commit 16ce41b7.

Reason: See
https://github.com/rails/rails/commit/16ce41b7f4449d6df15df30d69aef18da6510f36#commitcomment-14475125

- key was a poor choice of name. A key implies something that will
  unlock a lock. The concept is actually more like a 'lock identifier'
- mysql documentation calls this a 'lock name'
- postgres documentation calls it a 'lock_id'
- Updated variable names to reflect the preferred terminology for the database in
  question

Use latest byebug