Add ability to run replication protocol over redis. (#7040)

changelog.d/7040.feature

stubs/txredisapi.pyi

synapse/app/homeserver.py

synapse/config/homeserver.py

synapse/config/redis.py

synapse/python_dependencies.py

synapse/replication/tcp/client.py

synapse/replication/tcp/commands.py

synapse/replication/tcp/handler.py

synapse/replication/tcp/protocol.py

synapse/replication/tcp/redis.py

tests/replication/slave/storage/_base.py