Persist DHCP leases to a local database

Persist DHCP leases to a local database

Due to issues caused by dnsmasq restarts sending DHCPNAKs,
change Ieff0236670c1403b5d79ad8e50d7574c1b694e34 passed the
'dhcp-authoritative' option to dnsmasq. While this solved the
restart issue, it broke the multi-DHCP server scenario because
the dnsmasq instances will NAK requests to a server ID that
isn't their own.

Problem DHCP Request Lifecycle:

Client: DHCPDISCOVER(broadcast)
Server1: DHCPOFFER
Server2: DHCPOFFER
Client: DHCPREQUEST(broadcast with Server-ID=Server1)
Server1: DHCPACK
Server2: DHCPNAK(in response to observed DHCPREQUEST with other Server-ID)
          ^---Causes issues

This change removes the authoritative option so NAKs are not
send in response to DHCPREQUEST's to other servers. To handle
the original issue that Ieff0236670c1403b5d79ad8e50d7574c1b694e34
was inteded to address, this patch also allows changes to be persisted
to a local lease file.

In order to handle the issue where a DHCP server may be scheduled
to another agent, a fake lease file is generated for dnsmasq to start
with. The contents are populated based on all of the known ports for
a network. This should prevent dnsmasq from NAKing clients renewing
leases issued before it was restarted/rescheduled.

Closes-Bug: #1457900
Change-Id: Idc91602bf8c474467e596cbd5cbaa8898952c841