Fixes file page cache problem with long collection path

c5 fails to save or get cache files, because too long collection path makes too long page cache file name.

```
file_put_contents(/path/to/web/application/files/cache/pages/%/E/4/%E4%BA...%E3%81%A3.cache): failed to open stream: Invalid argument
```

Let's hash it like as Doctrine FileCache!