Fix some thread-safety warnings in MuonGeoModel

Hi,

this MR fixes all but one thread-safety warnings in MuonGeoModel. The last remaining warning is that MuonDetectorFactory001::getDetectorManager is declared const although it is not const. This warning cannot be avoided since the constness is inherited from GeoVDetectorFactory::getDetectorManager which cannot be easily changed. It closes ATLASRECTS-5255. Note that the warning won't cause MT issues since this method is called during initialisation.

Best, Nico