This significantly improves throughput of a peer relay server on Linux.
Server.packetReadLoop no longer passes sockets down the stack. Instead,
packet handling methods return a netip.AddrPort and []byte, which
packetReadLoop gathers together for eventual batched writes on the
appropriate socket(s).
Updates tailscale/corp#31164
Signed-off-by: Jordan Whited <jordan@tailscale.com>