Merge pull request #4796 from matrix-org/erikj/factor_out_e2e_keys

changelog.d/4796.feature

docs/workers.rst

synapse/app/client_reader.py

synapse/handlers/device.py

synapse/replication/slave/storage/deviceinbox.py

synapse/replication/slave/storage/devices.py

synapse/replication/slave/storage/push_rule.py

synapse/server.py

synapse/storage/deviceinbox.py

synapse/storage/devices.py

synapse/storage/end_to_end_keys.py