Lock calls to Reflex::Class::PathToBase

Instances of Reflex::Class are shared across threads. Calls to
Class::PathToBase caches values in a std::map for later use. Since
C;ass::PathToBase can be called from different threads, the map
needs to be locked. For simplicity, all Reflex::Class instances share
the same lock.
Merge pull request #13 from Dr15Jones/lockReflexClassPathToBase

Lock calls to Reflex::Class::PathToBase