Added test for an intermediate swappable model change in migration state.

refs #22563
Fixed #12118 -- Added shared cache support to SQLite in-memory testing.
Removed unused import.
Fixed #23450 -- Fixed transient failure of requests...test_far_expiration.
Revert "Updated some docs for the delayed deprecation of legacy table creation; refs #22340."

This reverts commit a2e3c9694823d6d451d6dd234e9d416600aacc68.

The deprecation was moved back to 1.9 in
61da5f3f02f34810aaa6fcddac3808318a5b95c4.
Removed db.backends methods which only call super().
Added ignore_warnings decorator

And removed Ignore*DeprecationWarningsMixin, now obsolete.
Thanks Berker Peksag and Tim Graham for the review.
Applied ignore_warnings to Django tests
Removed django.utils.dictconfig per deprecation timeline.
Removed django.utils.importlib per deprecation timeline.
Removed django.utils.tzinfo per deprecation timeline; refs #17262.
Removed django.utils.unittest per deprecation timeline.
Removed fallback suport for allow_syncdb() in database routers per deprecation timeline.
Removed backwards compatibility shims for "util" modules per deprecation timeline.

refs #17627.
Removed request.REQUEST per deprecation timeline; refs #18659.
Removed django.utils.datastructures.MergeDict per deprecation timeline; refs #18659.
Removed FastCGI support per deprecation timeline; refs #20766.
Removed django.utils.datastructures.SortedDict per deprecation timeline.
Removed ModelAdmin.declared_fieldsets per deprecation timeline; refs #18681.
Removed django.utils.functional.memoize per deprecation timeline.

refs #21351.