Unique Mapper Mode for RMC / RMS

[phspo]

Currently, few mappers dominate the map pool heavily, which in itself is not an issue. However, it could be an interesting variation of the commonly played challenges to avoid repetition of a mapper during a specific RMS/RMT/RMC run. This variation would result in a broader variety of maps being played and maybe present a nice alternative. This would require keeping track of already played mappers and using the current implementation it might lead to significant slowdown since many randomly fetched maps might belong to mappers that were already played in the current run. UI wise this could easily be implemented as a checkbox under "Advanced Settings".