dlz_bind9: Fix the named crash on reloading named

dlz_bind9: Fix the named crash on reloading named

When reloading zones, named first creates new zone instance and then shuts down
the old instance. Since ldb layer, keeps the same LDB open, talloc_free() on samdb
handle, causes talloc "access after use" error.

This patch keeps only single context (dlz_bind9_data) and uses reference counting
to decide when to actually free the context. Since samdb handle is reused, use
talloc_unlink() instead of talloc_free() on samdb handle.