[webrunner]: use latest key 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']

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