Cleanup PluginManager to avoid StaticAnalyzer warnings

The remaining statics in the PluginManager are protected by various mutex so are thread safe. Added the appropriate C++11 attribute to let the StaticAnalyzer know the code is fine.
Merge pull request #3183 from Dr15Jones/cleanupPluginManagerForStaticAnalyzer

FWCore/PluginManager -- Cleanup PluginManager to avoid StaticAnalyzer warnings