In order to make migration from 3.x apps easier, we should try to
convert
Hash instances to HashWithIndifferentAccess, to allow accessing values
with both symbol and a string. This is follow up to changes in 3c0bf043.

Currently, when saving a frozen record, an exception would be thrown
which causes a rollback. However, there is a bug in active record that
"defrost" the record as a side effect:

    >> t = Topic.new
    => #<Topic id: nil, ...>
    >> t.freeze
    => #<Topic id: nil, ...>
    >> t.save
    RuntimeError: can't modify a frozen Hash
    >> t.frozen?
    => false
    >> t.save
    => true

This patch fixes the bug by explictly restoring the frozen state on the
attributes Hash after every rollback.

Only constantize class_name once.

This reverts commit 23a98ffb.

[ci skip]

The fix commited in e2a070cf was returning the `new_target`, as
a try to return whatever user replaced association with. The
problem is, the resulting association target may be ordered
differently. In such case we want to return the target that
will be later used for that association.

Bug #6289

This reverts commit 8d7c38b8.

Reason: Lot of the additions aren't adding value.

[ci skip]

Record.from("(#{sub_query.to_sql})")    -> Record.from(sub_query)
Record.from("(#{sub_query.to_sql}) a")  -> Record.from(sub_query, :a)

Now that migrations support instance methods, we should
use the same instance rather than relying on delegation
to a global instance. This allows subclassing AR::Schema.