Create restart.sentinel after storing peer list

Otherwise, we risk to lose peer list when any failure happens after
the creation but before storing the list and as a consequence,
the weave container gets restarted.

prog/weaver/main.go

@@ -322,6 +322,7 @@ func main() {
 	if errors := router.InitiateConnections(peers, false); len(errors) > 0 {
 		Log.Fatal(common.ErrorMessages(errors))
 	}
+	checkFatal(router.CreateRestartSentinel())
 
 	// The weave script always waits for a status call to succeed,
 	// so there is no point in doing "weave launch --http-addr ''".

router/network_router.go

@@ -239,12 +239,16 @@ func (router *NetworkRouter) InitialPeers(resume bool, peers []string) ([]string
 		return storedPeers, nil
 	}
 
-	sentinel, err := os.Create("restart.sentinel")
-	if err != nil {
-		return nil, fmt.Errorf("error creating sentinel: %v", err)
-	}
-	sentinel.Close()
-
 	log.Println("Launch detected - using supplied peer list:", peers)
 	return peers, nil
 }
+
+func (router *NetworkRouter) CreateRestartSentinel() error {
+	sentinel, err := os.Create("restart.sentinel")
+	if err != nil {
+		return fmt.Errorf("error creating sentinel: %v", err)
+	}
+	sentinel.Close()
+
+	return nil
+}