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
Scripts/HoS: update Maiden of Grief
Core/Movement: Fixed vehicle passengers disappearing clientside

Closes #11411
Misc:

* Tools: Fixed a compile warning
* Commands: Fixed npcflag format in .npc info
Merge remote-tracking branch 'TrinityCore/master'