CHANGELOG-3.4: add "raftsnap" package
- Move `cmd/vendor` directory to `vendor` at repository root.
- Remove recursive symlinks in `cmd` directory.
- Now `go get/install/build` on `etcd` packages (e.g. `clientv3`, `tools/benchmark`) enforce builds with etcd `vendor` directory.
+- Rename `"github.com/coreos/etcd/snap"` to [`"github.com/coreos/etcd/raftsnap"`](https://github.com/coreos/etcd/pull/9211).
### Added(API)
