Improved NPC event processing.

Executing unique events (NPC::Label) no longer loops through all events.
Executing global event labels (::Label) also no longer loops through all
events. Global event labels are now mapped to list of only relevant NPC
events.
Cleaned up code related to clearing npc path db.
Merge pull request #56 from piotrhalaczkiewicz/master

Improved NPC event processing.