Merge pull request #231 from PrivateStorageio/224.renew_lease-is-gone

.circleci/config.yml

src/_zkapauthorizer/_storage_client.py

src/_zkapauthorizer/_storage_server.py

src/_zkapauthorizer/foolscap.py

src/_zkapauthorizer/storage_common.py

src/_zkapauthorizer/tests/fixtures.py

src/_zkapauthorizer/tests/strategies.py

src/_zkapauthorizer/tests/test_storage_protocol.py

src/_zkapauthorizer/tests/test_storage_server.py