Change the interface once more, so that the entry is stored at the same time as the cache


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/caching-agent-5126-2@37458 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb
Docstrings, test for interface


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/caching-agent-5126-2@37459 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb