Even more SRB2Kart netcode improvement ports

- Prevent connection timeout during the waiting gamestate from the last commit.
- Keep client connections alive during fades / other internal loops.
- More consistently timeout clients when they reach the end of BACKUPTICS.
- Dedicated servers will not run any game logic if no nodes are sending packets to it, to reduce CPU usage when there is no one interacting with your server.
  - Unlike SRB2Kart, the amount of time is configurable with the "dedicatedidletime" console variable. Setting this to 0 will disable this feature.
- CL_SendClientCmd uses exact packet types instead of magic number offsets.