! util: Swap Duration.Undefined by Duration.Inf, fixes #440

When confugarion is read using the pimped getDuration method the
mapping for "infinite" was Duration.Undefined, but Duration.Undefined !=
Duration.Undefined, which makes comparing any internal settings representation
containing a Duration in a sure failure. Duration.Inf make a lot more sense.
Merge pull request #441 from ivantopo/issues/440

! util: Swap Duration.Undefined by Duration.Inf, fixes #440