dns_view is detached from deleted zone too late

When zone is removed from the configuration, it keeps a reference to .view (and possibly .prev_view) until the zone is deleted. The removal can take a long time, and if there's a constant stream of rndc reconfig adding and removing zones, the otherwise dead views can be kept in memory for really long time leading to a memory bloat.

This symptom might also be encountered on a server with a Catalog Zone with a high rate of change.