Deprecate the behavior of AR::Dirty inside of after_(create|update|save) callbacks

We pretty frequently get bug reports that "dirty is broken inside of
after callbacks". Intuitively they are correct. You'd expect
`Model.after_save { puts changed? }; model.save` to do the same thing as
`model.save; puts model.changed?`, but it does not.

However, changing this goes much farther than just making the behavior
more intuitive. There are a _ton_ of places inside of AR that can be
drastically simplified with this change. Specifically, autosave
associations, timestamps, touch, counter cache, and just about anything
else in AR that works with callbacks have code to try to avoid "double
save" bugs which we will be able to flat out remove with this change.

We introduce two new sets of methods, both with names that are meant to
be more explicit than dirty. The first set maintains the old behavior,
and their names are meant to center that they are about changes that
occurred during the save that just happened. They are equivalent to
`previous_changes` when called outside of after callbacks, or once the
deprecation cycle moves.

The second set is the new behavior. Their names imply that they are
talking about changes from the database representation. The fact that
this is what we really care about became clear when looking at
`BelongsTo.touch_record` when tests were failing. I'm unsure that this
set of methods should be in the public API. Outside of after callbacks,
they are equivalent to the existing methods on dirty.

Dirty itself is not deprecated, nor are the methods inside of it. They
will only emit the warning when called inside of after callbacks. The
scope of this breakage is pretty large, but the migration path is
simple. Given how much this can improve our codebase, and considering
that it makes our API more intuitive, I think it's worth doing.

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