Add support for logging and stat collection related to IPv6 HR conflicts
This is related to #2419 (closed)
Investigate code paths related to IPv6 Host Reservation conflicts, and add a ALLOC_ENGINE_V6_DISCOVER_ADDRESS_CONFLICT
log message as well as v6-reservation-conflicts
and subnet[id].v6-reservation-conflicts
stats to reflect issues with allocating IPv6 Host Reservations.