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_bench_test.go

@@ -9,8 +9,8 @@ import (
 )
 
 func BenchmarkARPExchange(b *testing.B) {
-	const MTU = 1500
-	const frameSize = MTU + ethernet.MaxOverheadSize
+	const MTU = ethernet.MaxMTU
+	const frameSize = ethernet.MaxFrameLength
 	c1, c2 := new(StackAsync), new(StackAsync)
 	queryAddr := netip.AddrFrom4([4]byte{192, 168, 1, 2})
 
@@ -76,8 +76,8 @@ func BenchmarkARPExchange(b *testing.B) {
 }
 
 func BenchmarkTCPHandshake(b *testing.B) {
-	const MTU = 1500
-	const frameSize = MTU + ethernet.MaxOverheadSize
+	const MTU = ethernet.MaxMTU
+	const frameSize = ethernet.MaxFrameLength
 	const svPort = 8080
 	client, sv := new(StackAsync), new(StackAsync)
 	clconn, svconn := new(tcp.Conn), new(tcp.Conn)