De-prioritize pinging unreachable nodes (when using that mode)

We have "unreachable" status in addition to dead (if enabled).

Unreachable is used to keep trying even though it should be dead and waiting for another system confirm that yes we should confirm it dead.

That keep trying makes it the same as suspect today; while in reality it should be "less of a priority".
We could reflect this somehow in how we ping unreachables.

