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Use an example from a default Rails app (4.2.5) rather than an outdated one in the Configuring Rails Components section.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

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

Fixed a few grammar issues.

[ci skip] fix inconsistent indentation

[ci skip] Add CHANGELOG for #22300 (817c1825…

In b71e08f8 we started raising when nil or false was passed to merge to
fix #12264, however we should also do this for truthy values that are
invalid like true.

Test case for Issue #9015 - ignore_table and table_prefix at same time

Make `AR::SpawnMethods#merge!` to check an arg is a Proc

send normalized keys to the cache backends so they do not need to man…

Except keys of `build_record`'s argument from `create_scope` in initi…

Use proper syntax for class method reference in changelog

If argument of `build_record` has key and value which is same as
default value of database, we should also except the key from
`create_scope` in `initialize_attributes`.
Because at first `build_record` initialize record object with argument
of `build_record`, then assign attributes derived from Association's scope.
In this case `record.changed` does not include the key, which value is
same as default value of database, so we should add the key to except list.

Fix #21893.

Make the `config.static_index` option part of the `config.public_server` option

Also call it `public_server.index_name` so it'll make more sense.

Remove not needed `NATIVE_DATABASE_TYPES` entries

The string returned here will ultimately get used as a key of a hash in
the attribute set once the attributes are being built. When you give a
non-frozen string to `Hash#[]`, it will be duped. Be freezing we can
significantly reduce the number of times we end up allocating
`"user_id"`

This does not include any additional tests, as this should not have any
public facing implications. If you are mutating the result of
`Reflection#foreign_key`, please stop.