fix(server): avoid race conditions in Run/Stop

- don't close ready channel until PeerServer is listening.
  avoids possible panic in Stop() if PeerServer is nil.

- avoid data race in Run() (err variable was shared between 2 goroutines)

- avoid data race in PeerServer Start/Stop (PeerServer.closeChan)

test.sh

@@ -2,6 +2,9 @@
 
 . ./build
 
+go test -i ./etcd
+go test -v ./etcd -race
+
 go test -i ./http
 go test -v ./http -race