Fix particle spawners not visible since CSM spawner implementation (#8289)

Fix particle spawners not visible since CSM spawner implementation (#8289)

* Drop the ID mapper, use a big u64 instead. This will permit to resync server ids properly with the manager code
* Modernize some code parts (std::unordered_map, auto)
* generate id on client part on U32_MAX + 1 ids, lower are for server ids