[r=blake-rouse][bug=1657491][author=mpontillo] DHCP lease parser fixes.
* Fix improper handling of DHCP leases with a 'free' binding state,
and/or no 'ends' time.
* Fix lease parser to properly parse static host mappings with a '-'
in the mapping key (such as the entries that MAAS itself writes).
* Fix DNS zones to be updated when lease parser finishes and updates
the region.
* Fix lease parser to properly remove released leases.
* Fix traceback that could occur when the fast lease parser attempts
interpret a released lease.