Serialize Checkers PluginTypeManager to a dict

It seems likely that the multiprocessing module on Windows is not
capable of serializing an object with the structure that we have and
preserving the attributes we dynamically set on plugins (like the
FlakesChecker). To avoid issues like this with all plugins (although
we have only found this on Windows with the FlakesChecker), let's try
serializing the Checkers PluginTypeManager to a dictionary so that the
only object that a Queue is really trying to serialize/deserialize is
the FlakesChecker itself.

Related to #179