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.go

@@ -32,7 +32,6 @@ type Conn interface {
 	// message may fall on either side of a nonzero.
 	//
 	// Each [ipv6.Message.OOB] must be sized to at least MinControlMessageSize().
-	// len(msgs) must be at least MinReadBatchMsgsLen().
 	ReadBatch(msgs []ipv6.Message, flags int) (n int, err error)
 	// WriteBatchTo writes buffs to addr.
 	//