After this commit, returning `false` in a callback will display a deprecation
warning to make developers aware of the fact that they need to explicitly
`throw(:abort)` if their intention is to halt a callback chain.

This commit also patches two internal uses of AS::Callbacks (inside
ActiveRecord and ActionDispatch) which sometimes return `false` but whose
returned value is not meaningful for the purpose of execution.

In both cases, the returned value is set to `true`, which does not affect the
execution of the callbacks but prevents unrequested deprecation warnings from
showing up.

This commit changes arguments and default value of CallbackChain's :terminator
option.

After this commit, Chains of callbacks defined **without** an explicit
`:terminator` option will be halted as soon as a `before_` callback throws
`:abort`.

Chains of callbacks defined **with** a `:terminator` option will maintain their
existing behavior of halting as soon as a `before_` callback matches the
terminator's expectation. For instance, ActiveModel's callbacks will still
halt the chain when a `before_` callback returns `false`.

Add missing nodoc's
Change `assoc_klass` argument name to `association_klass`
Change `prev_reflection` argument name to `previous_reflection`
Change `prev` to `previous_reflection` in `#get_chain`
Switch use of `refl` and `reflection` in `#get_chain` so main parameter
is not abbreviated.
Add missing space in `#add_constraints`

This moves the `#type_caster` from the `aliased_table_for` and into the
initialize of the `alias_tracker`.

`yield` instead of relying on checking if the reflection is equal to the
`chain_head`.

Instead of initializing an empty connection use the base table name
instead. Split up and refactor `#create` to be 2 methods `#create` and
`#create_with_joins`. Removes the need to update the count by 1 on
initialzing a JoinDependency.

This moves `alias_candidate` out of the `ReflectionProxy` and into the
`AbstractReflection` so it is shared by all reflections. Change
`alias_name` to a method and and remove assignment in `#get_chain`.

After the refactorings we're only using the connection and not the alias
tracker anymore. This builds on commit 18019.

Reuse the already available `@connection` to reduce the surface area of
the alias tracker's API. We can then remove the `attr_reader` because
the connection is already available.

This makes the `#alias_name` more functional.

Move `RuntimeReflection` and `PolymorphicReflect` into Reflection. This
allows the methods to inherit from `ThroughReflection` and DRY up the
methods by removing duplicates.

The linked list lets us use a loop in `#add_constraints` and completely
remove the need for indexing the iteration becasue we have access to the
next item in the chain.

By concatnating the `ReflectionProxy` with the `chain` we remove
the need for `#construct_tables` because the `chain` is now in the
correct order (order of the chain DOES matter).

Putting the `#alias_name` into ReflectionProxy means we don't have to
cache the `#alias_name` globally anymore - it's not cached per query.

`is_first_chain`, `items` and `klass` are no longer beneficial and can
be called directly instead of via their assignments - because they are
each only used once.

Move method structure into reflection classes for accessibly on each
reflection rather than by traversing the chain.

The `RuntimeReflection` class allows the reflection to be accessed at
runtime - then we always know which reflection we are accessing in the
chain. The `#get_chain` method then allows us to recursively access the
chain through the `RuntimeReflection`.

`#constraints` builds a flattened version of `scope_chain` to
allow it to be accessible without requiring an index when iterating
over the `scope_chain`

It already treats the message

`visit_ChangeColumnDefinition` is the same "CHANGE column_name " + `visit_ColumnDefinition(o)`.

In most cases, `create_table_definition` called by table_name (the first
argument) only.

Incompatible to rounding behavior between MySQL 5.6 and earlier.

In 5.5, when you insert `2014-08-17 12:30:00.999999` the fractional part
is ignored. In 5.6, it's rounded to `2014-08-17 12:30:01`:

http://bugs.mysql.com/bug.php?id=68760

This constant may be define for auxiliar gems like rails-html-sanitizer
and these methods call will fail.

So we can change the arity later.

A few of the tests weren't testing anything of value. The IP Address
tests are testing the type, not behavior of the connection adapter.
There are two CVE regression tests which are important, but don't have a
good place to go, so I've left them alone for now, as they call `quote`
and the focus right now is removing `column` from `type_cast`

The column itself has no actual impact on the return value. These were
actually testing the behavior of the type object, which is sufficiently
covered elsewhere.

The behavior tested by the removed lines is sufficiently covered
elsewhere.

The string encoding test wasn't using the types for anything. The
boolean casting test included logic that should be in the tests for the
types, and the string test was legitimately not testing anything useful.

The validator has access to richer type information

SQL has mechanisms we can use to copy data from one table into another.

As we move towards removing the types from the column objects, any
remaining places which have access to richer type information should be
using it.

The column is no longer used for anything besides type casting, which is
what we're trying to remove from the column entirely.

This error only happens when the foreign key is missing.

Before this fix the following exception was being raised:

    NoMethodError: undefined method `val' for #<Arel::Nodes::BindParam:0x007fc64d19c218>

Now the message is:

    ActiveRecord::UnknownAttributeError: unknown attribute 'foreign_key' for Model.