EZP-20960: Contradictory error message in eZDFSFileHandlerMySQLiBackend::_fetch

Sometimes, the `eZDFSFileHandlerMySQLiBackend::_fetch()` reports an error that two files don't match, although the file sizes are the same.

```
eZDFSFileHandlerMySQLiBackend::_fetch:
Size (68) of written data for file 'var/ezflow_site/cache/expiry1225-1473899818tmp.php' does not match expected size 68
```

The reason for this is that the DFS filesize information is fetched the DFS-Database before the loop, while the local filesize is re-checked on every loop. In the meantime, the DFS filesize could have changed through another process.