Fix thread-safety warnings for MuonLayerHoughTool and MuonTruthSummaryTool
This MR makes some changes to MuonLayerHoughTool and MuonTruthSummaryTool to improve thread-safety and get rid of the MT warnings. Most importantly, MuonTruthSummaryTool was made thread-safe with mutexes, and as a result the handle to MuonTruthSummaryTool in MuonLayerHoughTool doesn't need to be mutable anymore.