[webrunner]: use newest timestamp in equeue.db to get last backup date

in some webrunner, there are 2 keys in equeue.db:

```
>>> print(db.keys())
['/srv/slapgrid/slappart18/bin//runner-importer', '/srv/slapgrid/slappart18/bin/runner-importer']
```

As you can see, the importer script path changed. But we don't care
about this, we always want to have the latest (= the newest) backup date.