MUC refactoring
This is a generic clean-up of the MUC code, that should not introduce (relevant) functional changes. This commit intends to lead up to more structural changes.
OF-2174: Reduce verbosity of authentication failures
When an anonymous or non-existing user tries to log in, a very alerting warning can be logged, when the authentication mechanism in use tries to make use of the user's salt (which does not exist). This needlessly alarms administrators. The verbosity should be turned down a notch or two.
