Tcp buffer bug fix (#79)

* start working on tracking down tcp buffer bug

* mtu refactor

* modularize test

* more precise testing

* tests fail, but is it the failure we are looking for?

* fix typo in espradio link (#76)

* implement a new backoff abstraction (#75)

* rewrite backoff api

* rewrite tcp.Conn.Write

* keep fixing small things

* much better Conn.Read implementation

* fix critical overflow bug in internal.ConnRWBackoff

---------

Co-authored-by: Joel Wetzell <jwetzell@yahoo.com>

x/xnet/xnet_arp_test.go

@@ -4,10 +4,12 @@ import (
 	"bytes"
 	"net/netip"
 	"testing"
+
+	"github.com/soypat/lneto/ethernet"
 )
 
 func TestARPLocal(t *testing.T) {
-	const mtu = 1500
+	const mtu = ethernet.MaxMTU
 	const seed = 1
 	s1, s2, c1, c2 := newTCPStacks(t, seed, mtu)
 	routerHw := [6]byte{1, 2, 3, 4, 5, 6}