Fix more cases of writing with slot_readv_and_testv_and_writev

src/_zkapauthorizer/_storage_client.py

src/_zkapauthorizer/_storage_server.py

src/_zkapauthorizer/storage_common.py

src/_zkapauthorizer/tests/__init__.py

src/_zkapauthorizer/tests/test_storage_protocol.py

src/_zkapauthorizer/tests/test_storage_server.py