Discovery: restore preference to latest unicast pings describing the same node

#5413 introduced a change where we prefer ping responses containing a master over those who don't. The same change changes the preference of acceptance if both pings have a master indication or if neither do.

 #7558 added new flag to the PingResponse which changes after a node has joined the cluster for the very first time. Giving preference to older pings cause the wrong value of this flag to be used.   This commit restores the preference to the original one.