Z39.50 stored credentials

Z39.50 stored credentials

* New non-IDL-accessible DB table for storing credentials
* API for applying credentials
* Additions to the Z39.50 configuration UI for applying and clearing
  credentials.
* At Z39.50 search time, if no creds are provided by the caller, but
  creds are configured in the database, creds from the DB are used to
  make the Z39 search call.
* Release notes included

Signed-off-by: Bill Erickson <berick@esilibrary.com>