refactor: rename Peer -> Instance

src/governor/cc.rs

src/main.rs

src/on_shutdown.rs

src/storage.rs

src/traft/error.rs

src/traft/mod.rs

src/traft/network.rs

src/traft/node.rs

src/traft/rpc/expel.rs

src/traft/rpc/join.rs

src/traft/rpc/mod.rs

src/traft/rpc/replication.rs

src/traft/rpc/sharding.rs

src/traft/rpc/update_peer.rs

src/traft/topology.rs

test/conftest.py

test/int/test_basics.py

test/int/test_expelling.py

test/int/test_joining.py

test/int/test_replication.py