Goal-seeking needs to control parameters for realtime modes

[alvestrand]

The RT mode needs to keep the lookahead parameter at zero to meet its requirements.
This means that the goal-seeker needs to know to avoid it - this may fit best if the ConfigurationFixups function of "Codec" takes a "mode" parameter - this requires keeping the name of the mode around, which argues for encapsulating the scoring function into an object having both a mode name and a score function.