Fixed #21012 -- New API to access cache backends.

Thanks Curtis Malony and Florian Apolloner.

Squashed commit of the following:

commit 3380495e93f5e81b80a251b03ddb0a80b17685f5
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sat Nov 23 14:18:07 2013 +0100

    Looked up the template_fragments cache at runtime.

commit 905a74f52b24a198f802520ff06290a94dedc687
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sat Nov 23 14:19:48 2013 +0100

    Removed all uses of create_cache.

    Refactored the cache tests significantly.

    Made it safe to override the CACHES setting.

commit 35e289fe9285feffed3c60657af9279a6a2cfccc
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sat Nov 23 12:23:57 2013 +0100

    Removed create_cache function.

commit 8e274f747a1f1c0c0e6c37873e29067f7fa022e8
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sat Nov 23 12:04:52 2013 +0100

    Updated docs to describe a simplified cache backend API.

commit ee7eb0f73e6d4699edcf5d357dce715224525cf6
Author: Curtis Maloney <curtis@tinbrain.net>
Date:   Sat Oct 19 09:49:24 2013 +1100

    Fixed #21012 -- Thread-local caches, like databases.