Merge pull request #4666 from matrix-org/erikj/register_login_split

changelog.d/4666.feature

docs/workers.rst

synapse/app/client_reader.py

synapse/handlers/register.py

synapse/replication/http/__init__.py

synapse/replication/http/login.py

synapse/replication/http/register.py

synapse/rest/client/v1/login.py

synapse/rest/client/v2_alpha/register.py

synapse/storage/registration.py

tests/rest/client/v2_alpha/test_register.py