Scripts/Misc: Initialize a couple of fields

(cherry picked from commit 548cc1cd2f31ca5fa767f148a086c76f8e9ef903)
Core/Player: Prevent updating visibility if player is not in world. Closes #13659
Update CONTRIBUTING.md
DB/BattleNet: Update wow build to 19243
Create INSTALL
Core/Scripts: log ScriptNames assigned in database without core script
Fixed build
Core: fixed few coverity issues and warnings
Core/DataStores: Refactored "gt" dbc handling. Wrapped these stores in a new GameTable class which will calculate correct dbc entry index based on given row/column as well as read number of rows/columns from client files instead of hardcoded GT_MAX_LEVEL
Core: fixed gcc build
Core/Packets: Add connection indexes for server packets
Fixed IOCP define for linux
Minor code style change in inventory loading
Compile warning fix
Merge branch '6.x' into Spells

Conflicts:
	src/server/game/DataStores/DBCStores.h
	src/server/game/Entities/Player/Player.cpp
	src/server/game/Entities/Unit/Unit.cpp
	src/server/game/Server/Protocol/Opcodes.cpp
	src/server/scripts/Spells/spell_druid.cpp