Broken by d1d7c86d

This will allow to set fixtures path in global config easier, it should
be not needed to use FIXTURES_PATH every time user wants to load
fixtures.

We discussed that the auto explain feature is rarely used.
This PR removes only the automatic explain. You can still display
the explain output for any given relation using `ActiveRecord::Relation#explain`.

As a side-effect this should also fix the connection problem during
asset compilation (#9385). The auto explain initializer in the `ActiveRecord::Railtie`
forced a connection.

make connection_url_to_hash a class method

This als prevents loading database.yml if it doesn't exist
but DATABASE_URL does

Rather than just raising a NoMethodError when copying the config, this
commit adds a warning message until either the rails-observers gem is
installed or the relevant config options are removed.

They was extracted from a plugin.

See https://github.com/rails/rails-observers

[Rafael Mendonça França + Steve Klabnik]

Most apps upgrading from 3.x will have options for mass assigment in
their application.rb and environments/*.rb config files. Rather than
just raising a NoMethodError when copying the config, this commit
adds a warning message until either the protected_attributes gem
is installed or the relevant config options are removed.

In the end I think the pain of implementing this seamlessly was not
worth the gain provided.

The intention was that it would allow plain ruby objects that might not
live in your main application to be subclassed and have persistence
mixed in. But I've decided that the benefit of doing that is not worth
the amount of complexity that the implementation introduced.

The new option allows any Ruby namespace to be registered and set
up for eager load. We are effectively exposing the structure existing
in Rails since v3.0 for all developers in order to make their applications
thread-safe and CoW friendly.

Get rid of ActiveModel::Configuration, make better use of
ActiveSupport::Concern + class_attribute, etc.

can be configured by setting config.active_record.migration. Setting to :page_load will raise an error on each page refresh if there are migrations that are pending. Setting to :page_load is defaulted in development for new applications. 

Active Record should be lazy loaded and this pull request
forced Active Record to always be loaded after initialization.

This reverts commit 8cd14c0b, reversing
changes made to 2f0c8c52.

minor