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 leader in leaderstats is always itself now, and is only set in https://github.com/coreos/etcd/pull/1730/files#diff-971f6af906b31cc86efd71aff56aa96bR269
so this is not correct to set the leader of server stats to it.

We are going to revisit stats part later. current stats is not enough for us to debug easily.