Merge pull request #68 from PrivateStorageio/67.stat_shares

src/_zkapauthorizer/_storage_client.py

src/_zkapauthorizer/_storage_server.py

src/_zkapauthorizer/foolscap.py

src/_zkapauthorizer/tests/strategies.py

src/_zkapauthorizer/tests/test_controller.py

src/_zkapauthorizer/tests/test_storage_protocol.py