Instead of std::map<std::string, boost::shared_ptr>, use std::unique_ptr as
the mapped_type, using emplace() to store new entries. This more correctly
captures the desired semantics: we have no intention of passing around the
pointers in the map, we just want the map to delete them on destruction.

Use std::function instead of boost::function.

(cherry picked from commit 7ba53ef82db5683756e296225f0c8b838420a26e)