API: clean up cryptokeys resource

API: clean up cryptokeys resource

Treat domain not found as a normal not found; treat all keys
absent for single resource request as a normal not found;
return an object instead of a single-valued list for single
resource requests and rename "content" to "privatekey"
and don't re-read the key from the backend.

Also share UeberBackend with DNSSECKeeper to avoid extra
backend connection.