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
Ack. Nowadays we have returned the current leader through ready here:
https://github.com/coreos/etcd/pull/1730/files#diff-971f6af906b31cc86efd71aff56aa96bR348
I want to update the leader info there in the running loop instead of before the main loop because the leader can change in the running process.