net/udprelay: use batching.Conn (#16866)

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>

net/batching/conn_default.go

@@ -19,3 +19,5 @@ var controlMessageSize = 0
 func MinControlMessageSize() int {
 	return controlMessageSize
 }
+
+const IdealBatchSize = 1