registry: add UnitHash to published UnitStates

This introduces a new field in UnitStates containing the string representing
the Hash of the associated Unit, to disambiguate between UnitStates published
with the same name but actually representing different Units.
@bcwaldon I want to explicitly check you're OK with this before landing this. (I would consider this temporary; once we finalise the `AgentReconciler` we can likely simplify the systemdUnitStateManager)