Implement lease6-bulk-apply command for the use of HA

In the HA world we're using leaseX-update command to carry lease updates to be partner and leaseX-del to carry information about the leases to be deleted. This works pretty well in v4 because there is usually a single address sent within the lease4-update. In the v6 world, however, this may be highly inefficient, because multiple addresses and/or prefixes may be allocated at the same time. It would be better if we had a command which aggregates all addresses and prefixes from the single client. This would result in one lease update transaction for one DHCP packet.

With this ticket, I am proposing to add the lease6-bulk-apply command which holds both leases to be deleted and leases to be updated/added.