This patch enables the helper program to detect event(s) triggered when
there is a change in lease length or expiry and client-id. This
transfers complete control of leases database to libvirt and obsoletes
use of the lease database file (<network-name>.leases). That file will
not be created, read, or written.  This is achieved by adding the option
--leasefile-ro to dnsmasq and passing a custom env var to leaseshelper,
which helps us map events related to leases with their corresponding
network bridges, no matter what the event be.

Also, this requires the addition of a new non-lease entry in our custom
lease database: "server-duid". It is required to identify a DHCPv6
server.

Now that dnsmasq doesn't maintain its own leases database, it relies on
our helper program to tell it about previous leases and server duid.
Thus, this patch makes our leases program honor an extra action: "init",
in which it sends the known info in a particular format to dnsmasq
by printing it to stdout.

The drawback of this change is that upgrade to this new approach does
not transfer the existing leases for the network if the leaseshelper
wasn't already used.