NMManager tries to assign unique route-metrics in an increasing manner
so that the device which activates first keeps to have the best routes.

This information is also persisted in the device's state file, however
we not only need to persist the effective route-metric which was
eventually chosen by NMManager, but also the aspired metric.

The reason is that when a metric is chosen for a device, the entire
range between aspired and effective route-metric is reserved for that
device. We must remember the entire range so that after restart the
entire range is still considered to be in use.

Fixes: 6a32c64d