Don't hard-code the name of the Logger to that of a particular subclass

The Logger of AbstractAuthenticationManager is now named for one of its concrete children, AuthenticationManagerImpl. This is confusing (although not necessarily incorrect, but that's a discussion about semantics), because log statements actually originating from AAM are rendered as coming from AMI.

This becomes more that a nuisance when you subclass AbstractAuthenticationManager. In this scenario, AuthenticationManagerImpl is never involved at any point, but still the Logger is named after it, so you're sent a wild goose chase trying to find the responsible code.

This change fixes the problem for extending classes, while preserving the old behaviour for AbstractAuthenticationImpl.