Allow per serial responder overrides

[rolandshoemaker]

Currently there is a bunch of code which allows the user to define a number of entries using only the serial + issuer + responders in the configuration file instead of just providing certificate files/a folder to watch.

I'm not 100% sure this feature is actually that useful, since in basically every case where you'd want to use the daemon you'd probably have a local copy of the certificate on hand.

See comment below.

[jsha]

I think no. The certificate should be the main unit of configuration. However, you may want a feature to override responder URL on a per-serial basis.