replace file system check with WF check

@drsnyder — regarding memcache, it depends on the actual implementation. PHP's native Memcache::set() returns false on both error and key expired. The `MemCachedClientforWiki` we use via `$wgMemc` behaves that way, too.

```
michal@cron-s5:~$ SERVER_ID=177 php /usr/wikia/slot1/current/src/maintenance/eval.php 
> var_dump( $wgMemc->set('a-non-existing-key', 42, 30) );
bool(true)

> var_dump( $wgMemc->get('a-non-existing-key'));
int(42)

> ^C
michal@cron-s5:~$ SERVER_ID=177 php /usr/wikia/slot1/current/src/maintenance/eval.php 
> var_dump( $wgMemc->get('a-non-existing-key')); // 10 seconds later
string(2) "42"

> ^C
michal@cron-s5:~$ SERVER_ID=177 php /usr/wikia/slot1/current/src/maintenance/eval.php 
> var_dump( $wgMemc->get('a-non-existing-key')); // 40 seconds later
bool(false)
```