Close channel of started servers even if embed server fails to start.
Close channel of started servers even if embed server fails to start.

Before this change:

```
gwmux, err = sctx.registerGateway([]grpc.DialOption{grpc.WithInsecure()})
if err != nil { return err; }
```

caused the Etcd.Serve() method finish without sctx.serversC channel being closed.
The code closing Etcd got stuck waiting for all the servers to be running.

Signed-off-by: Piotr Tabor <e4bb293449d6dc191d376a1c45c33ee02cbb0fe0@google.com>

ajityagaty/etcd,mqliang/etcd,ajityagaty/etcd,mqliang/etcd,etcd-io/etcd,ajityagaty/etcd,etcd-io/etcd,mqliang/etcd,etcd-io/etcd