Convert auth handler to async/await (#7261)

changelog.d/7261.misc

synapse/handlers/auth.py

synapse/handlers/device.py

synapse/handlers/register.py

synapse/handlers/set_password.py

synapse/module_api/__init__.py

tests/api/test_auth.py

tests/handlers/test_auth.py

tests/handlers/test_register.py

tests/utils.py