Use net functions for entities instead of networking entity ids

[Vurv78]

Lot of addons, including wiremod, used Read/WriteUInt to read entity id's to avoid getting a NULL entity while an entity is being transported onto another realm, but since now it networks a 13 bit integer, this could break existing behavior.

Not a direct bug report, but I feel like this is gonna break a lot of things if not dealt with.

Facepunch/garrysmod@00286d4

[Vurv78]

Doesn't seem like there's any actual bugs considering no reports, turning this into discussion to refactor some code that networks entity ids to instead use the proper entity functions (or at least change to 13 bits)