fix internal log rotation when check_interval > 0

As far as I understand (probably because of a typo like below) time since log rotation was last checked was calculated on a wrong way and it never happened if check_interval > 0.

```src/lager_file_backend.erl
@@ -132 +132 @@ handle_event({log, Message},
-            {ok,write(State, lager_msg:timestamp(Message), lager_msg:...
+            {ok,write(State, State#state.last_check, lager_msg:...
```

Hope I understand the author's intention correctly.

I think the issue is also present on adt-file-write branch.

I will try to add unit test with delayed mode (all tests currently use check_interval=0)