core: whenever we change state of a unit, force out PropertiesChanged bus signal

core: whenever we change state of a unit, force out PropertiesChanged bus signal

This allows clients to follow our internal state changes safely.

Previously, quick state changes (for example, when we restart a unit due
to Restart= after it quickly transitioned through DEAD/FAILED states)
would be coalesced into one bus signal event, with this change there's
the guarantee that all state changes after the unit was announced ones
are reflected on th bus.

Note we only do this kind of guaranteed flushing only for unit state
changes, not for other unit property changes, where clients still have
to expect coalescing. This is because the unit state is a very
important, high-level concept.

Fixes: #10185