Use native UPSERTs where possible (#4306)

.coveragerc

.gitignore

changelog.d/4306.misc

synapse/storage/_base.py

synapse/storage/client_ips.py

synapse/storage/engines/__init__.py

synapse/storage/engines/postgres.py

synapse/storage/engines/sqlite.py

synapse/storage/pusher.py

synapse/storage/user_directory.py

tests/storage/test_base.py

tests/test_server.py

tests/unittest.py

tox.ini