Convert replication code to async/await. (#7987)

changelog.d/7987.misc

synapse/handlers/register.py

synapse/replication/http/_base.py

synapse/replication/http/devices.py

synapse/replication/http/federation.py

synapse/replication/http/login.py

synapse/replication/http/membership.py

synapse/replication/http/presence.py

synapse/replication/http/register.py

synapse/replication/http/send_event.py

synapse/replication/http/streams.py