Began marking changes for monsters

These changes include storing monsters on shutdown, storing dead
characters and monsters, etc. Also fleshed out the MonsterManager's
internals a bit.

(cherry picked from commit af982710f6de050c09ef503cf2f8e1c9fdd34979)

server/monsters/monster_api.cpp

@@ -28,7 +28,7 @@
 static int setAvatar(lua_State* L) {
 	MonsterData* monster = static_cast<MonsterData*>(lua_touserdata(L, 1));
 	monster->SetAvatar(lua_tostring(L, 2));
-	//TODO: send an update to the clients?
+	//TODO: (1) send an update to the clients?
 	return 0;
 }