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']
```

We suppose that the 2nd key was used after the first key so we take the
timestamp of the last key.

/reviewed-on nexedi/slapos!578