feat: introduce crate::sync module and its APIs

src/lib.rs

src/rpc/ddl_apply.rs

src/rpc/lsn.rs

src/rpc/migration.rs

src/rpc/mod.rs

src/rpc/replication.rs

src/rpc/sharding.rs

src/rpc/sync.rs

src/schema.rs

src/sync.rs

src/traft/node.rs

test/conftest.py

test/int/test_cas.py

test/int/test_compaction.py

test/int/test_ddl.py

test/int/test_network_effects.py

test/int/test_snapshot.py

test/int/test_sql.py