The `config_readonly_open` function currently receives as input a
diskfile backend and will copy its entries to a new snapshot. This is
rather intimate, as we need to assume that the source config backend is
in fact a diskfile entry. We can do better than this though by using
generic methods to copy contents of the provided backend, e.g. by using
a config iterator. This also allows us to decouple the read-only backend
from the read-write backend.