core: enqueue unit earlier when state changes

core: enqueue unit earlier when state changes

Previously, we'd enqueue a unit to the dbus queue whenever the state
changed, after we processed the state change fully. This commit to the
beginning of the state change. This has the benefit that when the state
change causes a job to complete the unit is already in the dbus queue,
and thus we get the guarantee that any unit change can be sent out to
clients before the job change.