[9.16] Detach the views in zone_shutdown(), not in zone_free()

The .view (and possibly .prev_view) would be kept attached to the removed zone until the zone is fully removed from the memory in zone_free(). If this process is delayed because server is busy something else like doing constant rndc reconfig, it could take seconds to detach the view, possibly keeping multiple dead views in the memory. This could quickly lead to a massive memory bloat.

Release the views early in the zone_shutdown() call, and don't wait until the zone is freed.

(cherry picked from commit 13bb8212)

Closes #3801 (closed)

Backport of MR !7382 (merged)