Ref-count AuthScheme objects. Restructure the scheme-config relationship.

Ref-count AuthScheme objects. Restructure the scheme-config relationship.

* Schemes are now global permanent objects describing the components
  available. They hold the defaults for each auth mechanism (if any).

* Config are now temporary objects created by schemes and held in
  Auth::TheConfig until no longer needed. They hold all current config
  data about and auth mechanism.

TODO: ref-count the config items and clarify their relationship to
      requests, connections, and user details.