This includes a minimal in-memory implemenation which can be used by tests, and
adds the appropriate calls to the new interface to the storage server
implemenation.

avoids ambiguities and implementation problems of iso8601 duration strings

go back to aniso8601 dependency, since we no longer get anything distinctive
from isodate

tahoe-lafs 1.15 depends on attrs<20
something else pulls in attrs 21.2.0
pip fails to resolve the dependencies correctly in this case

So optimistic...

Cannot figure out how to have a VCS or other non-package-name dependency w/ install_requires.

We can put a plugin here later.