Mark PluginManager statics as thread safe

The singletons for the plugin system are required to be initialized before any threads are started, therefore the statics which hold those singletons are thread safe. This commit used the cms attribute to mark these as safe so we do not get a false positive from the static analyzer.
Merge pull request #3107 from Dr15Jones/markPluginManagerStaticsAsThreadSafe

Multithreading -- Mark PluginManager statics as thread safe