fix(LuaEngine/Reload): Await callbacks before reloading, to not crash on callback

[mostlynick3]

Eluna reloads close the Lua engine, then opens it again. Callbacks from queries run before it is closed have no reference after this, and lead to a crash once received. This schedules reloads until after all pending callbacks have been processed.

[Foereaper]

Instead of keeping track of expected callbacks you could just check whether or not the async callback processor is empty or not, pretty much exactly like we do here

https://github.com/ElunaLuaEngine/Eluna/blob/a1d26bdd09f9a02c4fdc00977dd69be095160068/LuaEngine.cpp#L991