etcdserver: initialise ServerStats with leader name if available, fixes  #561

When starting an EtcdServer set leader name in ServerStats if a leader already defined in LeaderStats. On a single machine cluster there are no AppendRequest calls to set leader name, while the machine is an active leader stats shows a default string as leader name. With this fix the name is correctly set.

Fixes #561
The server stats will be updated here, isn't it?
https://github.com/babo/etcd/blob/bug_561/etcdserver/stats/server.go#L108