Make a copy-able connection-config type to limit copy burden of isolation flags, etc

Right now, an increasingly large number of fields and flags are duplicated between port_cfg_t, listener_connection_t, and (say) entry_connection_t. Every field we add here needs to be added to every one of those types, and needs to be explicitly copied from each to the next during construction time.

It would make this code much more maintainable if there were a type that we just copied from object to object here.