[12790] CreatureLinking: Add const correctness
[12791] CreatureLinking: Improve Startup efficency

No need to use multiple select to check and obtain information about
db-data of an expectedly unique npc
[12792] CreatureLinking: Allow recursive CanSpawn checks

This will fix behaviour of mobs whose spawning is dependend on mobs whose
spawning is dependend on other mobs.

Close cmangos/issues#419