Allow Sharing of Zone Management

This blueprint proposes Designate is modified to allow it to share zone management with other tools.

Currently in the BIND driver, Designate needs to "own" a zone and can be the only tool to manage the zone. This is apparent every time a record is created, modified, or deleted when the content of the Designate database is synced to the zone file. Any changes to records in the zone file by other tools, such as nsupdate, are overwritten.