[Discovery] UnicastZenPing should also ping last known discoNodes

At the moment, when a node looses connection to the master (due to a partition or the master was stopped), we ping the unicast hosts in order to discover other nodes and elect a new master or get of another master than has been elected in the mean time. This can go wrong if all unicast targets are on the same side of a minority partition and therefore will never rejoin once the partition is healed.

Closes #7336