isBetterThan

"Smaller is better" ordering from bool operator<(node_entry const&):

std::make_tuple(!verified, rtt) < std::make_tuple(!rhs.verified, rhs.rtt)

i.e. verified nodes sort before unverified ones, and within the same verified-ness the lower RTT sorts first. Returns true if this is strictly better than other. Used by the replacement-selection logic in RoutingTable.