1. support metadata access rule define.
2. add benchmark tools.

1. block process until etcd connection is established
2. add health check api.

fix nodes argument bug.

1. fix mapping delete bug.

1. deprecated etcd v2 api
2. fix timer leak bug.

1. deprecate rsync api.
2. fix a concurrent bug.
3. format metad -version output.

fix CloseNotify called after ServeHTTP finished bug.

1. backend support etcd,etcdv3,local
2. support watch