Propagate opentracing contexts through EDUs (#5852)

changelog.d/5852.feature

docs/opentracing.rst

synapse/federation/federation_server.py

synapse/federation/sender/transaction_manager.py

synapse/federation/units.py

synapse/handlers/devicemessage.py

synapse/logging/opentracing.py

synapse/storage/devices.py

...pse/storage/schema/delta/56/add_spans_to_device_lists.sql