chore: remove obsolete Op variants, use dml in tests

src/lib.rs

src/traft/node.rs

src/traft/op.rs

src/traft/rpc/cas.rs

test/conftest.py

test/int/test_basics.py

test/int/test_couple.py

test/int/test_network_effects.py

test/int/test_uninitialized.py