refactor: improve join/update instance with cas

src/governor/mod.rs

src/lib.rs

src/luamod.rs

src/on_shutdown.rs

src/rpc/expel.rs

src/rpc/join.rs

src/rpc/update_instance.rs

src/schema.rs

src/storage.rs

src/traft/node.rs

src/traft/topology.rs

src/util.rs

test/int/test_cas.py

test/int/test_joining.py