Merge pull request #160 from PrivateStorageio/158.converged-upload-spending

src/_zkapauthorizer/_storage_client.py

src/_zkapauthorizer/_storage_server.py

src/_zkapauthorizer/eliot.py

src/_zkapauthorizer/tests/storage_common.py

src/_zkapauthorizer/tests/test_plugin.py

src/_zkapauthorizer/tests/test_storage_protocol.py