Reimplemented knotc flush/reload for specific zones.

Broken since the rrset API change.

Change-Id: If873ce3c9a86da019e09180f54d12f04c1bdb112
Fixed removing zones on reload.

Removing from zonedb read item value after it's been freed.
This could read a different zone or freed memory.

Change-Id: I5cd3844b4a9ac5030142d879ad872edc4694875a