lldp: refactor processing all lldp-neighbors

lldp: refactor processing all lldp-neighbors

Instead of replacing the whole hash with a new one (and all new by a new one,
LldpNeighbor instances), update the existing hash.

One point of this is that our process-all function requires less
comparisons and avoids duplicate work right earlier. E.g. if a neighbor
didn't change, we don't have to put it into a hash to compare later for
equality.

But more importantly, we preserve our LldpNeighbor instance instead
of recreating them all the time. Later, the LldpNeighbor will cache
the GVariant.