Support passing the schema name as a prefix to table name in
`ConnectionAdapters::SchemaStatements#indexes`. Previously the prefix would
be considered a full part of the index name, and only the schema in the
current search path would be considered.

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?`.

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

Apparently I managed to forget how similar the "tests passing" and
"no status reported" merge indicators look.

Note that the previous `stubs` in test_add_index wasn't working:
the method was still called, and just happened to return false.

See http://www.postgresql.org/docs/9.4/static/sql-dropindex.html
for more details.

Closes #21418.

Previously schema names were not quoted. This leads to issues when a
schema names contains a ".". Methods in `schema_statements.rb` should
quote user input.

[ci skip]

Currently the `#tables` method does not make use of the `name`
argument and always returns all the tables in the schema search
path. However the docs suggest different behavior.

While we should porbably adjust the implementation to provide this
behavior, let's make the docs right for now (also for `4-2-stable`) and
then implement the behavior on `master`.

Passing `:from` and `:to` to `change_column_default` makes this command
reversible as user has defined its previous state.

So, instead of having the migration command as:

    change_column_default(:posts, :state, "draft")

They can write it as:

    change_column_default(:posts, :state, from: nil, to: "draft")

Related with #20028.

Its already doc'ed in
activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

`exec_query` create `ActiveRecord::Result` instance. It is better to use
`select_value` instead of `exec_query` for performance.

Example:

    create_table :foos do |t|
      t.string :string_en, collation: 'en_US.UTF-8'
      t.text   :text_ja,   collation: 'ja_JP.UTF-8'
    end

Some databases like MySQL allow defining collation charset for specific
columns.

Its nodoc'ed for the other implementations, and doc'ed in the base
class, just like the other change_column* methods.

`change_column_null` is doc'ed only in
ActiveRecord::ConnectionAdapters::SchemaStatements, so it would make
sense to :nodoc: it elsewhere.

 - Added documentation for index_name_exists? and rename_index.
 - Also changed rails to \Rails in documentation of
   allowed_index_name_length.

It behaves in the same way that the abstract adapter.

[ci skip]

The type map was introduced in aafee233, but wasn't properly filled.

This mainly adjusts many locations, that expected strings instead of
integers or boolean.

add_pg_decoders is moved after setup of the StatementPool, because
execute_and_clear could potentially make use of it.

`[]` is a part of `sql_type`, so it is always necessary to respect to
array option when `type_to_sql` is called.

This reverts commit 1502caef.

The test suite for the mysql adapter broke when this commit was used
with MySQL 5.6.

Conflicts:
	activerecord/CHANGELOG.md

`quote_default_expression` and `quote_default_value` are almost the same
handling for do not quote default function of `:uuid` columns. Rename
`quote_default_value` to `quote_default_expression`, and remove
duplicate code.

`sql_type` is reused in `lookup_cast_type`. If making it a part of
`sql_type` when handled array option first, it isn't necessary to do
again.

The type from the column is never used, except when being passed to the
attributes API. While leaving the type on the column wasn't necessarily
a bad thing, I worry that it's existence there implies that it is
something which should be used.

During the design and implementation process of the attributes API,
there have been plenty of cases where getting the "right" type object
was hard, but I had easy access to the column objects. For any
contributor who isn't intimately familiar with the intents behind the
type casting system, grabbing the type from the column might easily seem
like the "correct" thing to do.

As such, the goal of this change is to express that the column is not
something that should be used for type casting. The only places that are
"valid" (at the time of this commit) uses of acquiring a type object
from the column are fixtures (as the YAML file is going to mirror the
database more closely than the AR object), and looking up the type
during schema detection to pass to the attributes API

Many of the failing tests were removed, as they've been made obsolete
over the last year. All of the PG column tests were testing nothing
beyond polymorphism. The Mysql2 tests were duplicating the mysql tests,
since they now share a column class.

The implementation is a little hairy, and slightly verbose, but it felt
preferable to going back to 20 constructor options for the columns. If
you are git blaming to figure out wtf I was thinking with them, and have
a better idea, go for it. Just don't use a type object for this.

The goal is to remove the type object from the column, and remove
columns from the type casting process entirely. The primary motivation
for this is clarity. The connection adapter does not have sufficient
type information, since the type we want to work with might have been
overriden at the class level. By taking this object from the column,
it is easy to mistakenly think that the column object which exists on
the connection adapter is sufficient. It isn't.

A concrete example of this is `serialize`. In 4.2 and earlier, `where`
worked in a very inconsistent and confusing manner. If you passed a
single value to `where`, it would serialize it before querying, and do
the right thing. However, passing it as part of an array, hash, or range
would cause it to not work. This is because it would stop using prepared
statements, so the type casting would come from arel. Arel would have no
choice but to get the column from the connection adapter, which would
treat it as any other string column, and query for the wrong value.

There are a handful of cases where using the column object to find the
cast type is appropriate. These are cases where there is not actually a
class involved, such as the migration DSL, or fixtures. For all other
cases, the API should be designed as such that the type is provided
before we get to the connection adapter. (For an example of this, see
the work done to decorate the arel table object with a type caster, or
the introduction of `QueryAttribute` to `Relation`).

There are times that it is appropriate to use information from the
column to change behavior in the connection adapter. These cases are
when the primitive used to represent that type before it goes to the
database does not sufficiently express what needs to happen. An example
of this that affects every adapter is binary vs varchar, where the
primitive used for both is a string. In this case it is appropriate to
look at the column object to determine which quoting method to use, as
this is something schema dependent.

An example of something which would not be appropriate is to look at the
type and see that it is a datetime, and performing string parsing when
given a string instead of a date.  This is the type of logic that should
live entirely on the type. The value which comes out of the type should
be a sufficiently generic primitive that the adapter can be expected to
know how to work with it.

The one place that is still using the column for type information which
should not be necessary is the connection adapter type caster which is
sometimes given to the arel table when we can't find the associated
table. This will hopefully go away in the near future.

If set to `if_exists: true`, it generates a statement like:

    DROP TABLE IF EXISTS posts

This syntax is supported in the popular SQL servers, that is (at least)
SQLite, PostgreSQL, MySQL, Oracle and MS SQL Sever.

Closes #16366.

When table has a composite primary key, the `primary_key` method for
sqlite3 and postgresql was only returning the first field of the key.

Ensures that it will return nil instead, as AR dont support composite pks.