Follow up to #19359 and avoid #22241.

This reverts commit 4d06ea9a, reversing
changes made to e9d15072.

Reason: This will break oracle-enhanced, see
https://github.com/rsim/oracle-enhanced/blob/3c42131db82b64ac41645db3affc6e4650289df6/lib/active_record/connection_adapters/oracle_enhanced_adapter.rb#L1254

`pool` in args is unused anymore. And `config` is used in all adapters.

Not needed for `Mysql2Adapter` and `AbstractMysqlAdapter`.

Not needed for `Mysql2Adapter` and `AbstractMysqlAdapter`.

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.

- 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

Reported on #21509, how views is treated by `#tables` are differ
by each adapters. To fix this different behavior, after Rails 5.0
is released, deprecate `#tables`.

And `#table_exists?` would check both tables and views.
To make their behavior consistent with `#tables`, after Rails 5.0
is released, deprecate `#table_exists?`.

The `native_database_types` only used in `TableDefinition` for look up
the default `:limit` option. But this is duplicated process with
`type_to_sql`. Passing `native_database_types` is not needed.

- Addresses issue #22092
- Works on Postgres and MySQL
- Uses advisory locks because of two important properties:
  1. The can be obtained outside of the context of a transaction
  2. They are automatically released when the session ends, so if a
  migration process crashed for whatever reason the lock is not left
  open perpetually
- Adds get_advisory_lock and release_advisory_lock methods to database
  adapters
- Attempting to run a migration while another one is in process will
  raise a ConcurrentMigrationError instead of attempting to run in
  parallel with undefined behavior. This could be rescued and
  the migration could exit cleanly instead. Perhaps as a configuration
  option?

Technical Notes
==============

The Migrator uses generate_migrator_advisory_lock_key to build the key
for the lock. In order to be compatible across multiple adapters there
are some constraints on this key.
- Postgres limits us to 64 bit signed integers
- MySQL advisory locks are server-wide so we have to scope to the
  database
- To fulfil these requirements we use a Migrator salt (a randomly
  chosen signed integer with max length of 31 bits) that identifies
  the Rails migration process as the owner of the lock. We multiply
  this salt with a CRC32 unsigned integer hash of the database name to
  get a signed 64 bit integer that can also be converted to a string
  to act as a lock key in MySQL databases.
- It is important for subsequent versions of the Migrator to use the
  same salt, otherwise different versions of the Migrator will not see
  each other's locks.

This issue was resolved by #21687 already. But re-add args by #18856.
`#tables` extra args was only using by `#table_exists?`. This is for
internal API. This commit will remove these extra args again.

Prior to this commit, Rails makes no differentiation between whether a
query uses bind parameters, and whether or not we cache that query as a
prepared statement. This leads to the cache populating extremely fast in
some cases, with the statements never being reused.

In particular, the two problematic cases are `where(foo: [1, 2, 3])` and
`where("foo = ?", 1)`. In both cases we'll end up quoting the values
rather than using a bind param, causing a cache entry for every value
ever used in that query.

It was noted that we can probably eventually change `where("foo = ?",
1)` to use a bind param, which would resolve that case. Additionally, on
PG we can change our generated query to be `WHERE foo = ANY($1)`, and
pass an array for the bind param. I hope to accomplish both in the
future.

For SQLite and MySQL, we still end up preparing the statements anyway,
we just don't cache it. The statement will be cleaned up after it is
executed. On postgres, we skip the prepare step entirely, as an API is
provided to execute with bind params without preparing the statement.

I'm not 100% happy on the way this ended up being structured. I was
hoping to use a decorator on the visitor, rather than mixing a module
into the object, but the way Arel has it's visitor pattern set up makes
it very difficult to extend without inheritance. I'd like to remove the
duplication from the various places that are extending it, but that'll
require a larger restructuring of that initialization logic. I'm going
to take another look at the structure of it soon.

This changes the signature of one of the adapter's internals, and will
require downstream changes from third party adapters. I'm not too
worried about this, as worst case they can simply add the parameter and
always ignore it, and just keep their previous behavior.

Fixes #21992.

Currently `tinyblob` is dumped to `t.binary "tiny_blob", limit: 255`.
But `t.binary ... limit: 255` is generating SQL to `varchar(255)`.
It is incorrect. This commit fixes this problem.

Current master branch includes many schema dumping improvements.
It extract these features to the appropriate files.

Current master branch includes many schema creation improvements in
MySQL. It extract these features to the appropriate file.

Current master branch includes many schema definition improvements in
MySQL. It extract these features to the appropriate file.

`pk_and_sequence_for` is implemented for PG and MySQL adapters (not
implemented for Sqlite3 adapter). But MySQL adapters are not using
`pk_and_sequence_for` already.

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)

`table_exists?` calls `tables` twice when passed `'dbname.tblname'` arg.
This change is that `table_exists?` execute only once query always and
extra args of `tables` is removed.

Example:

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

Currently in schema dumping, `create_table_info` query is called twice
for each tables. It means if 100 tables exists, the query is called 200
times. This change is that the query is called once for each tables in
schema dumping.

In the case of using `unsigned` as the type:

    create_table :foos do |t|
      t.unsigned_integer :unsigned_integer
      t.unsigned_bigint  :unsigned_bigint
      t.unsigned_float   :unsigned_float
      t.unsigned_decimal :unsigned_decimal, precision: 10, scale: 2
    end

Example:

    create_table :foos do |t|
      t.integer :unsigned_integer, unsigned: true
      t.bigint  :unsigned_bigint,  unsigned: true
      t.float   :unsigned_float,   unsigned: true
      t.decimal :unsigned_decimal, unsigned: true, precision: 10, scale: 2
    end

The **(11)** does not affect the storage size of the data type, which for an
INT will always be 4 bytes. It affects the **display width**.

http://www.tocker.ca/2015/07/02/proposal-to-deprecate-mysql-integer-display-width-and-zerofill.html

Bumps from `5.6` to `5.7`

Several changes were made in #21110 which I am strongly opposed to.
(this is what I get for going on vacation. :trollface:) No type should
be introduced into the generic `ActiveRecord::Type` namespace, and
*certainly* should not be registered into the registry unconstrained
unless it is supported by *all* adapters (which basically means that it
was specified in the ANSI SQL standard).

I do not think `# :nodoc:` ing the type is sufficient, as it still makes
the code of Rails itself very unclear as to what the role of that class
is. While I would argue that this shouldn't even be a super class, and
that MySql and PG's JSON types are only superficially duplicated (they
might look the same but will change for different reasons in the
future).

However, I don't feel strongly enough about it as a point of contention
(and the biggest cost of harming the blameability has already occured),
so I simply moved the superclass into a namespace where its role is
absolutely clear.

After this change, `attribute :foo, :json` will once again work with
MySQL and PG, but not with Sqlite3 or any third party adapters.

Unresolved questions
--------------------

The types that and adapter publishes (at least those are unique to that
adapter, and not adding additional behavior like `MysqlString` should
probably be part of the adapter's public API. Should we standardize the
namespace for these, and document them?