etcdserver: minor cleanup
etcdserver: fix TestDoProposalStopped test

We start etcd server in this test without the cluster. Sometimes it panics when
accessing the cluster. Most of the time it does not panic, since we can stop the
server fast enough before applying the first configuration change entry.
etcdserver: add StopNotify
Merge pull request #1714 from xiang90/stop

StopNotify