Implement a structured logging output system. (#5680)

.buildkite/docker-compose.py35.pg95.yaml

.buildkite/docker-compose.py37.pg11.yaml

.buildkite/docker-compose.py37.pg95.yaml

.buildkite/pipeline.yml

.gitignore

changelog.d/5680.misc

docs/structured_logging.md

synapse/app/_base.py

synapse/app/admin_cmd.py

synapse/app/appservice.py

synapse/app/client_reader.py

synapse/app/event_creator.py

synapse/app/federation_reader.py

synapse/app/federation_sender.py

synapse/app/frontend_proxy.py

synapse/app/homeserver.py

synapse/app/media_repository.py

synapse/app/pusher.py

synapse/app/synchrotron.py

synapse/app/user_dir.py