* Switch to keyword args where we can without breaking compat.
* Use add_table_options! for :options, too.
* Some code polish.

Comments are specified in migrations, stored in database itself (in its schema),
and dumped into db/schema.rb file.

This allows to generate good documentation and explain columns and tables' purpose
to everyone from new developers to database administrators.

For PostgreSQL and MySQL only. SQLite does not support comments at the moment.

See docs for PostgreSQL: http://www.postgresql.org/docs/current/static/sql-comment.html
See docs for MySQL: http://dev.mysql.com/doc/refman/5.7/en/create-table.html

Currently the results of `column.serial?` is not correct. For
`column.serial?` correctly working, initialize `column.table_name`
immediately.

This reverts commit 99801c6a.

Ultimately it doesn't matter whether `add_index` or `t.index` are used
in the schema dumper in any meaningful way. There are gems out there
which hook into the old behavior for things like indexing materialized
views. Since the reverted commit doesn't seem to add much benefit,
there's no reason for us to break these gems.

This PR introduces a key/value type store to Active Record that can be used for storing internal values. It is an alternative implementation to #21237 cc @sgrif @matthewd.

It is possible to run your tests against your production database by accident right now. While infrequently, but as an anecdotal data point, Heroku receives a non-trivial number of requests for a database restore due to this happening. In these cases the loss can be large.

To prevent against running tests against production we can store the "environment" version that was used when migrating the database in a new internal table. Before executing tests we can see if the database is a listed in `protected_environments` and abort. There is a manual escape valve to force this check from happening with environment variable `DISABLE_DATABASE_ENVIRONMENT_CHECK=1`.

These new methods are used from the Active Record model layer to
determine which relations are viable to back a model. These new methods
allow us to change `conn.tables` in the future to only return tables and
no views. Same for `conn.table_exists?`.

The goal is to provide the following introspection methods on the
connection:

* `tables`
* `table_exists?`
* `views`
* `view_exists?`
* `data_sources` (views + tables)
* `data_source_exists?` (views + tables)

Example:

    create_table :barcodes, primary_key: ["region", "code"] do |t|
      t.string :region
      t.integer :code
    end

The schema_migrations table name is configurable. We should use this value when checking for ignored table names when dumping schema instead of a hardcoded value.

Now in sqlite3, mysql and mysql2 adapters, SchemaDumper dump a view as
a table. It is incorrect behavior. This change excludes a view in
schema.rb.

If the adapter supports indexes in create table, generated SQL is
slightly more efficient.

Example:

    create_table :foos do |t|
      t.string :string_utf8_bin, charset: 'utf8', collation: 'utf8_bin'
      t.text   :text_ascii,      charset: 'ascii'
    end

If it is not a default primary key, correctly dump the type and options.

Technically changes the API, as it will allow any object which responds
to `===`. Personally, I think this is more flexible.

* Add private method primary_key_for, which more clearly shows that the
expected return value is nil when a primary key is not found.

* Consistent whitespace
* Remove unnecessary parentheses

Gems which wish to tie into ActiveRecord::SchemaDumper need to
duplicate this logic currently. [Foreigner] is one such example, as is a
library I'm currently working on but which hasn't been released yet:

    def tables_with_foreign_keys(stream)
      tables_without_foreign_keys(stream)
      @connection.tables.sort.each do |table|
        next if ['schema_migrations', ignore_tables].flatten.any? do |ignored|
          case ignored
          when String; table == ignored
          when Regexp; table =~ ignored
          else
            raise StandardError, 'ActiveRecord::SchemaDumper.ignore_tables accepts an array of String and / or Regexp values.'
          end
        end
        foreign_keys(table, stream)
      end
    end

[Foreigner]: https://github.com/matthuhiggins/foreigner/blob/master/lib/foreigner/schema_dumper.rb#L36-L43

Extract the skip logic to a method, making it much simpler to follow
this same behavior in gems that are tying into the migration flow and
let them dump only tables that aren't skipped without copying this block
of code. The above code could then be simplified to:

    def tables_with_foreign_keys(stream)
      tables_without_foreign_keys(stream)
      @connection.tables.sort.each do |table|
        foreign_keys(table, stream) unless ignored?(table)
      end
    end

It also, in my opinion, simplifies the logic on ActiveRecord's side, and
clarifies the intent of the skip logic.

For example, if use pgcrypto extension in PostgreSQL 9.4 beta 1, where
uuid-ossp extension isn't available for moment of writing, and thus to
use a gen_random_uuid() method as a primary key default.

In this case schema dumper wasn't able to correctly reconstruct
create_table statement and lost primary key constraint on schema load.

Fixes #16111.

The name of the foreign key is not relevant from a users perspective.
Using random names resolves the urge to rename the foreign key when the
respective table or column is renamed.

respect `table_name_prefix` and `table_name_suffix`.

Fixes #12489

These options make it easier to change the config from
ActiveRecord::Base to use something else inside of the SchemaDumper.