These 3 auth plugins all have a data store they fetch the reference
password or hash from. They then match the attemped password or hash
against the reference. This consolidates the latter portion (validating
the password/hash) into Auth.pm.
* less duplicated code in the plugins.
* Pass validation consistently handled for these 3 plugins.
* less work to create new auth plugins
Also caches the CRAM-MD5 ticket. It could also cache user/pass info if
this was desirable.
