Implementation of MSC2314 (#6176)

changelog.d/6176.feature

synapse/federation/federation_server.py

synapse/federation/transport/server.py

sytest-blacklist

tests/federation/test_complexity.py

tests/federation/test_federation_sender.py

tests/federation/test_federation_server.py

tests/handlers/test_typing.py

tests/replication/slave/storage/_base.py

tests/replication/tcp/streams/_base.py

tests/storage/test_roommember.py

tests/unittest.py

tests/utils.py