Make the isolated tests run on JRuby

As there is no forking on JRuby, we need to spawn sub-processes to make
the tests run in isolation.

Previously, we were defining globally env variables and running the test
file through backticks and delete these variables once the test ran.

Now, we simply rely on IO.popen as this is cross-platform and the env
variables are available during the child-process execution only so there
are no race conditions.

[Ben Browning & Robin Dupret]
Move `attributes_before_type_cast` to `AttributeSet`
`only_integer` of `NumericalityValidator` now allows procs and symbols
Encapsulate knowledge of type objects on `ActiveRecord::Result`

Attempting to reduce the number of places that care about the details of
how type casting occurs. We remove the type casting of the primary key
in `JoinDependecy`, rather than encapsulating it. It was originally
added for consistency with
https://github.com/rails/rails/commit/40898c8c19fa04442fc5f8fb5daf3a8bdb9a1e03#diff-06059df8d3dee3101718fb2c01151ad0R211,
but that conditional was later removed in
https://github.com/rails/rails/commit/d7ddaa530fd1b94e22d745cbaf2e8a5a34ee9734.

What is important is that the same row twice will have the same value
for the primary key, which it will.
Move behavior of `read_attribute` to `AttributeSet`

Moved `Builder` to its own file, as it started looking very weird once I
added private methods to the `AttributeSet` class and the `Builder`
class started to grow.

Would like to refactor `fetch_value` to change to

```ruby
self[name].value(&block)
```

But that requires the attributes to know about their name, which they
currently do not.
synchronize Active Record deprecations / removals with 4.2 release notes

[ci skip]

/cc @chancancode @zzak
sync deprecations and removals with 4.2 release notes. [ci skip]

Noticed that there was no section about Action View and added the heading.
Notable changes need to be compiled.

/cc @chancancode @zzak
docs, fix BacktraceCleaner code example. Closes #15911. [ci skip]
Merge pull request #15846 from sgrif/sg-attributes-before-type-cast

Move `attributes_before_type_cast` to `AttributeSet`

Conflicts:
	activerecord/lib/active_record/attribute_set.rb
	activerecord/test/cases/attribute_set_test.rb
Merge pull request #15847 from sgrif/sg-encapsulate-result-types

Encapsulate knowledge of type objects on `ActiveRecord::Result`
Merge pull request #15868 from sgrif/sg-uninitialized-attributes

Move behavior of `read_attribute` to `AttributeSet`

Conflicts:
	activerecord/lib/active_record/attribute_set.rb
	activerecord/test/cases/attribute_set_test.rb
Encapsulate the creation of `Attribute` objects

This will make it less painful to add additional properties, which
should persist across writes, such as `name`.

Conflicts:
	activerecord/lib/active_record/attribute_set.rb
Require shellwords since it is dependecy of this file

Closes #15919
Merge pull request #15834 from rmehner/allow_proc_and_symbol_for_only_integer

Allow proc and symbol as values for `only_integer` of `NumericalityValidator`
Make the apply_inflections method case-sensitive

Since d3071db1, the apply_inflections method check if the downcased
version of a string is contained inside the "whitelist" of uncountable
words. However, if the word is composed of capital letters, it won't be
matched in the list while it should.

We can't simply revert to the previous behavior as there is a
performance concern (benchmarked over /usr/share/dict/words):

Before d3071db1  135.610000   0.290000  135.900000 (137.807081)
Since d3071db1   22.170000    0.020000  22.190000  ( 22.530005)
With the patch   22.060000    0.020000  22.080000  ( 22.125771)

Benchmarked with http://git.io/aFnWig

This way, the solution is to put the down-case version of words inside
the @uncountables array.
Merge pull request #15845 from robin850/jruby-isolation

Make the isolated tests run on JRuby
`Attribute` should know about its name

This allows using polymorphism for the uninitialized attributes raising
an exception behavior.
Merge pull request #15870 from sgrif/sg-attribute-name

`Attribute` should know about its name
Deprecate automatic counter caches on has_many :through

Reliant on https://github.com/rails/rails/pull/15747 but pulled to a
separate PR to reduce noise. `has_many :through` associations have the
undocumented behavior of automatically detecting counter caches.
However, the way in which it does so is inconsistent with counter caches
everywhere else, and doesn't actually work consistently.

As with normal `has_many` associations, the user should specify the
counter cache on the `belongs_to`, if they'd like it updated.
Merge pull request #15754 from sgrif/sg-deprecate-hmt-counter-cache

Deprecate automatic counter caches on has_many :through