Add a tag to the cachedir

[mwichmann]

Since there are now two files to make when a cachedir is created, use the temporary-dir -> rename technique.

CacheDir tests no longer pre-create the cache directory, they should be verifying it's created properly upon request (one unit test still makes sure passing an existing empty directory works, too).

There are no externally visible changes (except the new tag file if someone goes and explicitly looks, but that's an "implementation detail", not part of the behavior).

Contributor Checklist:

• [X] I have created a new test or updated the unit tests to cover the new/changed functionality.
• [X] I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• [X] I have updated the appropriate documentation