Fix race condition when handling unknown packets.

[modmuss50]

Fixes #3505

This was a fun one to figure out, this change ensures that custom payloads are always relaesed on the network thread, and no further processing of them happens on the main thread.

This caused a rare race codition where a user might be disconnected from a server sending unknown packets with the following error: io.netty.util.IllegalReferenceCountException: refCnt: 0

[christopherplaysminecraft]

Thankful that it got fixed so fast, this has been annoying me for more than two weeks now.

I have found this playing BedWars on the Hypixel Server.
The server sends custom badlion:timers packets which allow the client to display information about the game (Time until new resources spawn, Game Events, etc.) and it does this even if your client doesn't even support those features.

This doesn't cause any issues, typically.