DB/Spells: remove duplicate/invalid entries from spelldifficulty_dbc
* For new entries: use spellid0 as id to avoid conflicts with dbc file
DB/Spells: correct more spelldifficulty_dbc entries
* add log message to find wrong entries
Core/Threading: Handle CMSG_ZONEUPDATE only in WorldSession update - changing zones triggers changing chat channels, what is not a thread safe operation
Merge branch 'master' of https://github.com/TrinityCore/TrinityCore