Integrate presence from hotfixes (#3694)

changelog.d/3694.feature

docs/workers.rst

synapse/app/_base.py

synapse/app/frontend_proxy.py

synapse/app/synchrotron.py

synapse/config/server.py

synapse/federation/transaction_queue.py

synapse/handlers/initial_sync.py

synapse/handlers/presence.py

synapse/handlers/sync.py

synapse/rest/client/v1/presence.py

tests/app/__init__.py

tests/app/test_frontend_proxy.py

tests/rest/client/v1/test_presence.py

tests/rest/client/v2_alpha/test_sync.py

tests/unittest.py

tests/utils.py