Exit immediately after SIGTERM is there are no signals running.

Cf. https://bugs.torproject.org/24875

client/snowflake.go

@@ -241,11 +241,8 @@ func main() {
 		ln.Close()
 	}
 	snowflakes.End()
-	for n := range handlerChan {
-		numHandlers += n
-		if numHandlers == 0 {
-			break
-		}
+	for numHandlers > 0 {
+		numHandlers += <-handlerChan
 	}
 	log.Println("snowflake is done.")
 }

server/server.go

@@ -393,10 +393,7 @@ func main() {
 	for _, server := range servers {
 		server.Close()
 	}
-	for n := range handlerChan {
-		numHandlers += n
-		if numHandlers == 0 {
-			break
-		}
+	for numHandlers > 0 {
+		numHandlers += <-handlerChan
 	}
 }