Add locking around std::map and std::unordered_map

[gregmedd]

We've encountered several places where concurrent access to maps is possible, but no locking is provided. Neither std::map or std::unordered_map are thread safe, so locks will need to be added.

One option for handling this is by adding a "safe" map wrapper that leverages a std::shared_mutex to allow parallel read-only access to the map (safe) while restricting modification to exclusive access modes. I've put a proposed PR into up-cpp with SafeMap and SafeUnorderedMap.