rafthttp: log before receiving snapshot

Database snapshot can be as large as 5GB. It is reasonable
to log before receiving it. Or the user might not know what
is happening and why etcd starts to use IO intensively.

rafthttp/http.go

@@ -169,6 +169,7 @@ func (h *snapshotHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	plog.Infof("receiving database snapshot [index:%d, from %s] ...", m.Snapshot.Metadata.Index, types.ID(m.From))
 	// save incoming database snapshot.
 	if err := h.snapshotter.SaveDBFrom(r.Body, m.Snapshot.Metadata.Index); err != nil {
 		msg := fmt.Sprintf("failed to save KV snapshot (%v)", err)